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- Herausgeber: Bentham Science Publishers
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Plant diseases in ornamental, aromatic and medicinal crops are common in occurrence, and account for up to 12-20% yield losses. Diseases of Ornamental, Aromatic and Medicinal Plants is a unique reference aimed to fulfil knowledge gaps on this subject. Nine chapters cover major diseases and nematode problems in important ornamental plants (cut flowers, potted flowers, flowering geophytes, and turfs), medicinal plants (tuberous and non-tuberous) and aromatic plants.
The book explains the etiology, symptoms, disease cycle, economic importance, distribution, and management of respective plants with advanced and sustainable approaches. The book is intended as a key resource for students, botanists and academics in plant science courses. It also serves as a quick guide for horticulture professionals and gardeners dealing with plant diseases and nematode infestations on premises.
Readership
Students, botanists and academics in plant science courses (undergraduate, graduate and postgraduate levels); horticulture professionals and gardeners.
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Seitenzahl: 597
Veröffentlichungsjahr: 2024
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FOREWORD
Plant diseases in ornamental, aromatic, and medicinal crops are common in occurrence and account for up to 12-20% yield losses. The book “Diseases of Ornamental, Aromatic and Medicinal Plants” by Prof. M. R. Khan and Dr. Z. Haque presents exclusive and comprehensive information on the major diseases and nematode problems in important ornamental plants (cut flowers, potted flowers, and flowering geophytes, turfs), medicinal plants (tuberous and non-tuberous) and aromatic plants. The present book aims to fill the knowledge gap and provides up-to-date relevant information in one place. It embodies nine chapters highlighting the aetiology, symptoms, disease cycle, economic importance, distribution, and management aspects with advanced and sustainable approaches. The present book shall serve as an important reference source to UG/PG students, academics, professionals, gardeners, scientists, researchers, and extension personnel in universities and institutes, bureaus, directorates, research stations, etc. dealing with plant diseases and nematode problems in ornamental, aromatic and medicinal plants. I wish to congratulate the editors of the book and contributors of the chapters for putting together their efforts to bring out this volume.
PREFACE
The cultivation of ornamental plants such as cut flowers, potted flowers, and flowering geophytes has become an emerging commercial sector due to the continuously rising demand for flowers throughout the world. This sector also provides enough opportunities for employment generation as well as foreign earnings. The global trade of florists' crops has been projected to be more than $100 billion annually. Medicinal plants are essential to human life and are utilized in both ancient and modern medical practices all over the world. India is a rich habitat for medicinal and aromatic plants, and accounts for about 9.18 lakh tonnes of production annually. Similarly, aromatic plants assume eminence and economic significance for containing high-value aromatic compounds. However, diseases in these plants are major factors, that destroy around 10-20% of crop produce during pre-harvest and post-harvest stages. A book presenting exclusive and comprehensive information on “Diseases of Ornamental, Aromatic and Medicinal Plants” is lacking in the market, as a result, potential readers are deprived of up-to-date relevant information. The present book aims to fulfill the gap of knowledge and embodies nine chapters covering major diseases and nematode problems in ornamental (such as, anthurium, azalea, begonias, caladium, carnation, china aster, chrysanthemum, coleus, cyclamen, daffodils, gardenia, gerbera, gladiolus, lily, marigold, poinsettia, tulip, turfgrasses, etc.), aromatic (such as, basil, citronella, eucalyptus, Geranium, mint, rose, orchid, tuberose, etc.) and medicinal crops (such as, aloe vera, ashwagandha, asparagus, isabgol, liquorice, noni, patarchur, safed musli, rosa grass, turmeric, etc.), highlighting the aetiology, symptoms, disease cycle, economic importance, distribution, and management aspects with advanced and sustainable approaches. The present book shall serve as an important reference source to UG/PG students, academics, professionals, scientists, researchers, kitchen gardeners, horticulturalists, and extension personnel dealing with plant diseases in the above crops.
List of Contributors
Major Diseases of Cut Flowers
Harjot Singh Sidhu1,*,Deepak Kumar2Abstract
Flower cultivation at the commercial level has attained immense significance in the area of crop diversification and contributes a lot to the cultivator's economy. With the worldwide boom in the floriculture trade, the production of high-quality and disease-free flowers has become an important challenge in commercial floriculture. Various biotic and abiotic factors affect flower production in open fields and controlled environments. Among the cut flowers, anthurium, carnation, china aster, chrysanthemum, gerbera, marigold, orchid, rose, etc. are covered in this chapter with regard to their plant diseases. Anthracnose, black spot, botrytis blight, crown rot, leaf spot, powdery mildew, mosaic, root-rot, rust, twig blight, stem necrosis, stump rot, white rust, wilt etc. are important diseases of the above cut flowers that shall be discussed in detail and suitable management strategies shall also be presented in the chapter.
INTRODUCTION
Floriculture involves the growing of loose-cut flowers with stems and long leaves, dried flowers, decorative plants, nurseries, flower seeds and propagative materials. The cultivation of cut flowers has become an emerging sector due to multiple uses, satisfying the aesthetic needs of the people, employment-generating, a source of higher income for people and facilitating more foreign earnings. In addition, flowers are used as raw material in the manufacturing of essence, perfumes, medicines and confectioneries. Cut flowers are cultivated worldwide and the largest producer of cut flowers is China followed by India and the USA. However, by value, the largest importer and exporter is the Netherlands. In India, cut flowers are grown in an area of around 0.32 mha with the production of 1.96 MMT and 0.82 MMT (FAOSTAT, 2022).
Cut or loose flowers such as anthurium, carnation, chrysanthemum, gerbera, marigold, orchid, rose and some other annuals are extensively grown to fulfil the demands of regional and national markets around the world. Cultivation of flowers and ornamental crops are viewed as a growing field but the export and import of flowers and other related products have come across many infrastructural problems like inadequate cooling systems during transportation, bad storage facilities and poor road conditions, especially in developing countries.
Other constraints with the production and handling of commercial cut flowers are shortage of trained manpower and poor phytosanitation. Various biotic and abiotic factors also affect cut flower production under open fields and protected conditions. Flowers are mostly grown in monoculture and so there are more chances to be attacked by various plant pathogens such as fungi, bacteria, viruses, phytoplasma, plant-parasitic nematodes etc. Sometimes, extensive economic loss to floriculture occurs due to disease occurrence. The qualitative and quantitative loss of flowers due to diseases greatly affect the economy of the farmers/growers and their products are rejected in the floriculture market. Important diseases associated with cut flowers are enlisted in Table 1. Major diseases of economically important cut flowers are described under:
DISEASES OF AZALEA
Anthracnose
Aetiology
Anthracnose of anthurium is caused by Colletotrichum gloeosporioides (Penz.) Sacc. The disease is also known as black nose or spadix rot.
Symptoms
The spadix's individual flowers are the main targets of the disease. The infection begins as little, black spots that subsequently become larger. Under damp conditions, a general rot of the entire spadix may happen in the advanced stages.
Biology and Epidemiology
The fungus Colletotrichum gloeosporioides produces an enormous amount of asexual spores in open acervuli on the flowers of anthurium. The disease may deteriorate during the rainy season because moisture facilitates spores germination, and other infection processes. The fungus can also invade petioles and pedicles (Aragaki et al., 1968). Acervuli formed on the dead leaves and stems, are carried by wind currents, irrigation water, and raindrop splashes to other plants.
Management
The warm, humid weather favours the occurrence of anthracnose. Hence, during the favourable weather, the crop should be properly watched and taken care of. Infection is decreased by growing anthuriums in a protected structure to keep the rain off the blossoms. When the disease crosses the financial threshold, fungicidal sprays should be used. Mancozeb or thiophanate methyl sprays applied on a weekly or biweekly basis are found effective.
Root Rot and Vascular Wilt
Aetiology
The root rot and vascular wilt of anthurium are incited by Rhizoctonia solani Kuhn.
Symptoms
Rot and discolouration on the roots and lower stems of anthurium plants are important symptoms caused by Rhizoctonia solani. The leaf blight and root decay may occur in extremely damp conditions under severe infection. R. solani infection causes other symptoms similar to those produced by Pythium and Phytophthora. However, a distinguishing characteristic between Pythium and Phytophthora infections and Rhizoctonia infections is the presence of distinctive mycelial threads in rotting roots.
Biology and Epidemiology
Rhizoctonia forms sclerotia (resting stage) in the form of compact mycelial mats that can withstand low temperatures and desiccation, allowing the fungus to live for a longer time in the soil without a host. These tiny, erratic brownish sclerotial bodies mix with soil or adhere to the surface of pots greenhouse beds. (Norman and Ali, 2012). Anthurium plants from damp soils and poorly oxygenated roots are more prone to Rhizoctonia infection.
Management
Avoid soils that are wet and ensure proper drainage. Plants can be grown on raised benches. Spraying fungicides viz., thiophanate methyl, flutolanil and fludioxonil repeatedly given with a gap of two to three weeks can manage the disease.
Root-Rot
Pythium splendens is the causal agent of the anthurium root rot. Infected plants exhibited reduced plant height, smaller leaves and blooms, as well as an overall lack of vigour. In severe infestations, every root may have withered. Being a typical oomycete, Pythium produces motile zoospores, and needs unrestricted access to water to thrive and spread the disease. The disease development is favoured by water-saturated soils. The disease cycle is presented in Fig. (1). Hence, management, is recommended. To control the disease, planting should be done in well-drained soil. Soil application with dimethomorph, mefenoxam, fluopicolide, etc. may effectively reduce the infestation level (Arora et al., 2021).
Fig. (1)) Disease cycle of Root Rot of Anthurium.Bacterial Wilt
Aetiology
Ralstonia solanacearum Smith is the causal agent of bacterial wilt of anthurium that is caused by (Yabuguchi, 1995).
Symptoms
The initial symptoms resemble with water stress. Plants' lower leaves begin to turn yellow, and their petioles grow floppy. The bacteria quickly invade the vascular region, turning the leaf veins and stems bronze in colour. The copious, cream to brown bacterial oozing can be seen in cross-sections of the stem or petiole along with vascular browning. Even after additional irrigation, plants are unable to recover. When the disease worsens causing necrotic leaves and stems, plants die (Norman and Ali 2012).
Biology and Epidemiology
The disease disseminates through contaminated water, soil, equipment, shoes or farm workers contacted with Ralstonia solanacearum (Norman and Ali, 2012). The bacteria are easily transmitted by infected cuttings and can persist for a long time in the contaminated soil. The creation of specialized immunodiagnostic and molecular assays has enhanced the detection of very low populations of the bacterium in the soil and potting material (Paret et al., 2010; Kubota et al., 2011). When the weather is chilly, plants may not show any symptoms, giving bacteria time to colonize other plants as latent infestations (Norman and Ali, 2012).
Management
To halt the spread of disease and to eradicate it from anthurium production unit, pathogen-free propagative material must be used in conjunction with a rigorous cleanliness programme. Phosphorous acid-containing fungicides shield another plant from new infections, but they lose their potency once the pathogen has invaded the entire plant system (Norman et al., 2006).
Bacterial Blight
Aetiology
The bacterial blight in anthurium, is incited by Xanthomonas axonopodis pv. dieffenbachiae.
Symptoms
At the initial initial stage of the disease, irregularly shaped water-soaked lesions bordered by slight yellowing on the underside surface of leaves are formed. These spots become darkened and are encircled by the yellow zone at the advanced stages. Systemic infection is characterized by chlorosis and wilting followed by a rapid decline in plant growth and eventually death of the entire plant.
Biology and Epidemiology
When sprinkler or rainstorm strikes the diseased leaves, the bacteria are disseminated to healthy plants. The disease also spreads through infested soil, cutting instruments, workers, etc. as well as through aerosols (Fig. 2). Latently infected propagative materials are the primary source for transmission, and the presence of the bacteria can be determined in these materials using certain immunodiagnostic techniques (Alvarez et al., 1991). Even while plants seem healthy, the extensive contamination of planting benches and subsequent crop death make the impact difficult to assess (Norman and Alvarez, 1996). In tissue-cultured microplants, the bacteria can persist as latent infections and triple indexing is required to guarantee that the bacteria are eradicated (Norman and Alvarez, 1994). However, pathogen-free plants that become infected on the benches of greenhouse may colonize the entire plant without displaying any symptoms (Ayin et al., 2016).
Management
The use of healthy propagative-materials free of pathogens is strongly recommended. In order to decrease disease outbreaks, shade-house temperatures can be lowered through aeration or anthurium production can be moved to cooler temperatures (18-24°C) with high elevations. It is advised to avoid planting too closely together and to take strict hygienic precautions, including removing any damaged leaves. It is recommended to perform three to four sprays of 200 ppm of streptomycin sulphate and 200 ppm of tetracycline at 10 to 12 days apart. As soon as symptoms of the disease first arise, the first spray should be administered.
DISEASES OF CARNATION
Fusarium Wilt
Aetiology
The fungus, Fusarium oxysporum f. sp. dianthi Snyder & Hansen causes carnation wilt.
Fig. (2)) Disease cycle of Bacterial Blight of Anthurium.Symptoms
The plants infected with F. oxysporum f.sp. dianthi show yellowing of foliage and production of crookneck shoots. The infected stems are softened so as to be easily crushed. Stem, when cut open, show brown zonation or striping at the vascular region. In severe infestation, the whole plant wilts and collapses in a very short time.
Biology and Epidemiology
Fusarium that lives in the soil can infect young or older plants at any point of the plant growth by entering the vascular tissues of the roots through either unwounded or wounded roots (Ben-Yephet and Shtienberg, 1997). The xylem tissues become brownish in colour as the colonisation on the stem spreads upward. The fungus creates pale pink masses of micro- and macroconidia on the colonized surface of the root and stem tissues when the plant dies (Fig. 3; Brayford, 1996). Additionally, the fungus reduces resistant chlamydospores that allow for long-term survival. F. oxysporum f. sp. dianthi can also thrive well in greenhouses benches, shoes and wood used to support benches. The spores can survive for a very long time and can spread through the air, water, soil, infected-cuttings, and tainted clothing, tools and farm equipments (Rattink, 1977). The fungus gnats also carry F. oxysporum.
Management
Drenching of soil with copper oxychloride (0.4%) and spraying with Bavistin (0.1%) reduce the disease. Deep summer ploughing of fileds and soil solarization of potted plants effectively reduce the disease. Biological agents such as Trichoderma harzianum, Pseudomonas fluorescens, Bacillus subtilis, Streptomyces sp. and non-pathogenic isolates of Fusarium are found to be effective against the Fusarium wilt disease. Neem-based formulations have also been recorded to be effective against this disease (Chandel and Tomar, 2008).
Stem/Root Rot
Aetiology
Stem or root rot of carnation is predominantly caused by Phytophthora nicotianae var. parasitica. Several other species of Phytophthora like P. capsici and P. cryptogea are also reported to cause this disease (Nakkeeran et al., 2018).
Fig. (3)) Disease cycle of carnation wilt caused by Fusarium oxysporum f.sp. dianthi.Symptoms
Under high moisture conditions, the fungus attacks the root and collar portion of the stem at the soil level which later results in wilting. The leaves get discoloured and start drying up from the bottom upwards.
Biology and Epidemiology
Both the asexual and sexual phases of the P. nicotianae life cycle are present. Hyphae, sporangia containing zoospores and chlamydospores are produced during the asexual stage (Fig. 4). The zoospores can swim in the film of soil water and infect stem and roots of other plants. Chlamydospores (resting spores), have thick walls and are typically formed near the terminals or in the centre of hyphae. The formation of oospores in P. nicotianae, which is predominately heterothallic, depends on A1 and A2 mating types. These oospores may endure extended durations in plant debris and soil because of their extremely strong wall. They can grow and generate sporangia in temperatures between 10 to 35 °C, with 25–30 °C being the ideal range. Wet seasons, excessive watering, and poor soil drainage are other factors that favour zoospore formation and plant infestation (Meng et al., 2014). These traits are shared by other Phytophthora species that have been seen in carnations.
Fig. (4)) Disease cycle of foot rot of carnation. [Modified from: https://www.wikiwand.com/en/ Phytophthora_nicotianae].Management
Avoiding excess soil moisture is an important step in the management strategy. Soil treatment with Fosetyl-AI and Metalaxyl are effective against Pythium and Phytophthora. Application of Trichoderma harzianum has been found to reduce the disease incidence by 70% (Nakkaran et al., 2018).
Leaf Spot
Aetiology
Leaf spot of carnation is caused by the fungus, Alternaria dianthi Stevens & Hall.
Symptoms
The fungus incites light brown lesions with a purplish-brown border on stems and leaves. The lowest leaves are infected first, and the disease progresses upwards. On enlargement, the lesions merge and result in the blighting and premature death of leaves.
Biology and Epidemiology
Spores of A. dianthi spread are disseminated by the wind, water, tools and animals. To start growing and infecting the leaves and stem, the pathogen needs unrestricted water. The fungus can persist in weeds or perennial crops that are vulnerable to it (Fig. 5; Mamgain et al., 2013). Long-term storage of the cuttings in cool chambers may be the cause of their decay (English and Kinthan, 1974).
Management
Removal of infected leaves is recommended to reduce the amount of inoculum in the field to prevent its spread. Foliar spray of Carbendazim or Dithane M-45 is effective in minimizing disease losses.
Botrytis Blight
Aetiology
Botrytis blight/Gray Mould/bud rot/blossom blight of carnation is caused by Botrytis cinerea Pers.: Fr.
Symptoms
Under high humid conditions, the fungus causes water-soaked flecking of the outer petals and gradually the entire flower is affected. The disease is very common during the stage of cut-carnation.
Fig. (5)) Disease cycle of Alternaria leaf spot of carnation caused by Alternaria dianthi.Biology and Epidemiology
The fungus in the form of mycelium, conidia and sclerotia can thrive on a variety of plant species either as a pathogen or saprophytes. Sclerotia are thought to be the pathogen's primary survival structure (Elad et al., 2007). The conidia are the primary component of dispersal and can be spread throughout a plant surface by wind, insects or rain splashes (Fig. 6).
Fig. (6)) Disease cycle of gray mould of carnation.The growth of B. cinerea is favoured by high ambient humidity. Following the spore germination, the fungus enters into the flower petals through injuries or natural openings (Gibson et al., 2014). An increase in ethylene production from infected tissues is associated with the progression of the disease (Elad and Eversen, 1995). Ethylene increases in the senescence of carnation blooms, which increases susceptibility to B. cinerea blight (Elad, 1988).
Management
Soil drenching and spraying of plants with Bavistin (0.1%) minimize disease intensity. Nakkuran et al. (2018) reported Mycostop, a biofungicide (Streptomyces griseoviridis) to control the disease.
Fusarium Rot
Aetiology
The Fusarium branch rot, is also known as stem rot, basal rot, stub dieback, cutting rot and pink disease, etc. Some 4-5 species of Fusarium have been reported to be associated with the disease. Important ones are F. graminearum Schwabe; F. culmorum (Smith) Saccardo; F. avenaceum (Fries) Saccardo; F. verticillioides (Saccardo) Nirenberg; and F. proliferatum (Matsushima) Nirenberg.
Symptoms
The disease occurs due to the injuries caused by continuous harvesting. The typical rotting can be seen on the stem at soil level or slightly higher on the infected plants. The plants wilt and later die.
Biology and Epidemiology
Pathogens may thrive in environments that are favourable for plant development. The cutting and flower collection may create wounds, which exposes tissue surfaces for entry of the pathogen. Disease propagation is further facilitated by lack of proper hygiene (Kalc Wright et al., 1997). On diseased stubs, the mycelium grows voluminously in conditions of high relative humidity. Although stub dieback affects plants at all stages of their development, 2-year-old plants suffer the greatest losses when they are at the harvesting stage of their flowers. This may be due to physiological age or because of dense foliage canopy that contributes to maintaining a high relative humidity (Nelson et al., 1975). In summer, when it is difficult to regulate the greenhouse climatic conditions that encourage disease growth, epidemics of basal rot and stem rot can emerge. The application of nitrogen-rich fertiliser to plants during the summer may result in lush, soft vegetative growth, which may facilitate the emergence of Fusarium stem rot (Dorworth and Tammen, 1969). Fusarium spp. can develop resistant chlamydospores that allow them for long-term survival and can spread aerially by microconidia and macroconidia. As with many other hosts, F. graminearum may thrive on plant waste.
Management
Since, Fusarium is a soil-borne, pathogen, soil fumigation and soil solarization can effectively reduce the inoculum level and may prove effective in controlling the disease. Spraying of Benomyl or Bavistin (0.1%) also controls the disease.
Fasciation
Aetiology
Fasciation or leafy gall of carnation is caused by Rhodococcus fascians (formerly Bacterium fascians, Corynebacterium fascians, Phytomonas fascians and Pseudobacterium fascians).
Symptoms
Carnations commonly exhibit fasciation as reduction in the size of flowers, leaves, and stems emerging from the infected stem. The typical symptoms include excessively abnormal proliferation of leaves and flowers development of leaf galls on leaves and wounds on stems, as a result of the hyperplastic growth of the damaged tissues (Kado and Kado, 2010).
Biology and Epidemiology
Rhodococcus fascians isolates that are highly pathogenic carry a sizable conjugative plasmid with at least three pathogenicity-related genes (Putnam and Miller, 2007). On the surface of aerial plants, R. fascians develops epiphytically and is shielded by a bacterial slime layer. The bacterial surface creates signals that start the development of symptoms, such as the creation of a leafy gall or the activation of preexisting meristems. Water splashing from sprinkler irrigation or rain causes the infection to spread. Dissemination depends on a lot of moisture. Low-humidity areas with semi-arid climates do not favour the disease development. The pathogen is seed borne and persists in soil and propagative materials. Through wounds, the bacterium colonizes intercellular spaces (Cornelis et al., 2001). The growers run the risk of passing the bacterium along with the cuttings and don't realize that the plants are diseased. The bacterium can survive on plant surfaces for several months before causing symptoms (Putnam and Miller, 2007). The symptoms induced by R. fascians and those caused by the use of plant growth hormones, are identical, making diagnosis challenging. R. fascians isolation and inoculation into a susceptible host are required to confirm the diagnosis and establish pathogenicity. The bacteria can be quickly and precisely detected using the, loop-mediated isothermal amplification (LAMP) and PCR techniques have been developed (Serdani et al., 2013).
Management
An efficient method of preventing bacterial fasciation is to practise good cleanliness and use plants free of pathogens. Do not propagate from diseased plants; instead, remove and discard them together with any deformed tissue or prune them. When R. fascians-antibiotics are used, the disease is treated and shoot growth returns to normal (Kado, 2010). Chemicals containing copper can also be used to control the disease. Although it is unknown if insects play a part in the spread of disease in the wild, aphids have been shown to be capable of doing so in laboratory settings (Putnam and Miller, 2007), hence it is advised to use chemical or biological methods to control insect populations.
DISEASES OF CHINA ASTER
Botrytis Blight
Botrytis blight, often known as grey mould, is a disease that affects many flower plants and other commercial crops across the world, causing significant damage during pre and post harvest stages (Horita and McGovern, 2018). A large outbreak of grey mould in China aster was seen during rainy season in farmed areas in Japan (Nishikado and Inoue, 1956). The fungal infection in the seed of China aster has been recorded in Poland (Rymar, 1984).
Aetiology
Botrytis blight or Gray mould is caused by the fungus, Botrytis cinerea Pers.
Symptoms
Brown blemishes appear first on the edges of immature leaves. The sores spread quickly, blighting whole leaves. Blighted tissues spread to the stems and petioles. Lesions can girdle stems, causing a dark discolouration at or above the site of infection. The whole plant wilts and dies when stems near the soil line become infected or diseased. Under the chilly, damp condition, the pathogen also causes petal blight. The profuse grey mould appears due to and development of masses of hyphae and spores of B. cinerea on leaves, stems and flowers (Horita and McGovern, 2018).
Biology and Epidemiology
Spores of Botrytis cinerea are airborne and infects approximately 200 different hosts, including all types of flowers. Infected plant detritus can support the fungus as a pathogen or saprophyte. Small, black stress-resistant sclerotia are also produced by B. cinerea. Infected tissues with sclerotia and mycelia can survive long periods of poor conditions for the fungus. Quiescent mycelia/sclerotia become active and germinate when disease-promoting conditions return, producing conidia which disseminated by wind and rain. Cool, damp environments are ideal for B. cinerea infection (Horita and McGovern, 2018).
Management
Plants and the soil surface should be free from dead and decaying leaves and flowers. To limit the period of leaf wetness in greenhouse culture, eliminate overhead watering or, if this is not possible, irrigate early in the day. Plant spacing should be increased to improve ventilation and prevent leaf wetness. Bertus (1967) found that treating the seeds with aerated steam (53 °C for 30 min) was effective in controlling B. cinerea infection in china aster. Carbendazim, benomyl and thiophanate methyl were reported to significantly suppress the seed infestation with Botrytis sp. (Rymar, 1984). In floral crops, however, B. cinerea has developed broad resistance to thiophanate methyl and iprodione (Daughtrey et al., 2000). In China and Poland, the fungicide treatment with azoxystrobin and D-limonene is natural and is effectively used to control B. cinerea (Nawrocki, 2013).
Fusarium Root-rot and Wilt
The root rot and wilt have been quite prevalent in China aster in Australia, New Zealand, Germany, Japan, Poland, the United States etc. (Horita and McGovern, 2018). Disease occurrences of up to 71.2% have been documented in the Connecticut and Florida states of the USA (Elmer and McGovern, 2013).
Aetiology
The root-rot and wilt disease is incited by Fusarium oxysporum f. sp. callistephi (Beach) Snyder & Hans and other Fusarium sp.
Symptoms
Damping-off is the first symptom that appears at the seedling stage. The cotyledons wither and the seedlings topple over as the stems rot at the soil level. The infected seedlings have suppressed growth with browning in the lower leaves. The seedlings wilt and die later. Seedlings develop brown lesions at the stem bases at the transplant stage, with black streaks extending to the top stems. When infected stems are sliced longitudinally, vascular discolouration is obvious. On the stem bases of dead and dying plants, white mycelia and masses of salmon-coloured spores appear (Horita and McGovern, 2018).
Biology and Epidemiology
The fungus produces three types of spores, microconidia and macroconidia, and chlamydospores. The chlamydospores are thick-walled and survive under adverse conditions to keep the fungus live in the absence of aster plants for many years. All F. oxysporum f. sp. callistephi isolates are highly pathogenic to China aster (Rataj-Guranowska et al., 2007). Although F. oxysporum f. sp. dianthus does not cause damage to China aster, but it may make the infection of F. oxysporum f. sp. callistephi highly severe (Orlicz-Luthardt et al., 2000). Four races of F. oxysporum f. sp. callistephi have been identified based on the differential pathogenicity to certain cultivars of China aster (Armstrong and Armstrong, 1971). The fungus inoculum survives in the infected seeds or decaying plant material in the soil (Neergaard, 1977). The disease thrives at warm temperatures (27 °C), in acidic soils, etc. The fungus can spread by infested implements, footwear, etc.
Management
The fungus was successfully eradicated from seedlings using sodium hypochlorite (Elmer and McGovern 2013) benomyl, and Carbendazim, and thiophanate methyl were reported to be effective in treating aster seed infected with Fusarium sp. (Rymar, 1984). The transplanting trays are thoroughly disinfected with chemicals or steam before reuse. It is critical to utilise pathogen-free transplants. Ammonia-based nitrogen fertilisers should be avoided. Raising the pH of the soil to 6.5-7.0 was found effective against the pathogen. Soil with the application of methyl bromide + chloropicrin, 1,3-dichloropropene + chloropicrin, or metam sodium has been found very effective against the disease. Elmer and McGovern ('2013') Soil solarization using transparent 0.025 mm-thick polyethene mulch for 3-4 weeks, which reduced the wilt incidence by 73.4% (Huang and Sun, 1991). Treatment with the biocontrol Pythium oligandrum and the fungicide azoxystrobin was found effective in suppressing F. oxysporum infection of China aster plants (Nawrocki, 2013).
Rhizoctonia Stem and Root-rot
Aetiology
Stem and root-rot or blight of China aster are incited by, Rhizoctonia solani. The China Aster isolates of R. solani belong to the AG-2-2 hyphal anastomosis group (Ito et al., 2006). The disease has been reported from Finland, Japan and the USA (Horita and McGovern, 2018).
Symptoms
First, light brown lesions appear on the stems at the soil level, which later get enlarged, and girdle the stems. The lesions spread to the root. The plants become stunted, experience wilting, and dies later. Some times, the blight symptoms may develop on the leaves touching the soil. At this stage, the dark brown hyphae of the fungus appear on the stems close to the soil. Blakish sclerotia are formed on the stem under root or soil (Horita and McGovern, 2018).
Disease Cycle
Rhizoctonia solani is an important pathogen that infects most cultivated crops and weeds all over the world. The fungus can be found in plant debris or soil as hyphae and sclerotia, and becomes active under warm and humid conditions. Seed infection has also been found in China aster (Crosier 1968).
Management
The diseased plants must be removed as soon as possible. Excessive watering and high heat should be avoided wherever feasible. Fumigation, steam, or soil solarization, as well as prophylactic fungicide treatment, are effective options for soil disinfestation. Pre-plant soil application of Trichoderma spp. should be followed by spraying the seedling base of Trichoderma, thiophanate methyl, and iprodione, which effectively controls the disease (Koi, 2016). Combined soil application of thiophanate methyl with a talc-based formulation of Pseudomonas fluorescens was also found effective in enhancing the germination of seeds and in reducing the root rot incidence in China aster (Babu et al., 2013).
Aster Yellows
Aetiology
Aster yellows are caused by a phytoplasma, Candidatus Phytoplasma asteris (Lee et al., 2004). In China and Myanmar, a new phytoplasma strain, 16SrII, has been discovered (Win et al., 2011). The disease aster yellow has been reported to occur in Canada, Germany, Japan, USA (Horita and McGovern, 2018).
Symptoms
The phytoplasma symptoms in china aster include yellowing of foliage, stunting of entire plants, and yellow-green discolouration of flowers. In Myanmar, floral virescence (flower greening) has also been reported (Win et al., 2011).
Disease Cycle
The aster leafhopper, Macrosteles fascifrons transmits the pathogen (Babadoost, 1988). In temperate locations, the aster yellows become infected by leafhoppers that overwinter on diseased host plants.
Management
Diseased plants must be removed as soon as possible. Also, vulnerable hosts (including weeds) must be eradicated around china aster producing areas. Reflective mulches can be used to confuse vectors in field-grown asters. To keep leafhoppers out of your greenhouse, physical barriers like fine mesh nets or screens must be employed.
DISEASES OF CHRYSANTHEMUM
Alternaria Leaf Spot
Aetiology
The leaf spot and blight of chrysanthemum is caused by Alternaria spp.
Symptoms
Brown to black, rounded to oval spots on the leaves and petals are the typical symptoms of leaf spot disease (Domnguez-Serrano et al., 2016). Necrotic leaves may develop if a disease continues to worsen. Lesions on infected flower petals range from 1 to 3 millimetres (0.04 to 0.12 inches) in diameter and range in severity from reddish brown to completely necrotic (Engelhard, 1970). Floral symptoms resemble with those caused by Botrytis, Helminthosporium,etc.
Biology and Epidemiology
Infected plant debris, water splashes, and wind can transmit disease (Chase, 2005). High relative humidity favours the progression of the disease because it facilitates the germination and infection of the spores.
Management
It is important to utilise pathogen-free planting material. Plants engineered to express the harpin gene are resistant to Alternaria leaf spot and bloom earlier than their non-transgenic counterparts (Xu et al., 2011). Alternaria leaf blight can be prevented by the application of biofertilizers such as vermicompost, paddy straw,
neem powder, cow dung, and fish meal, and biocontrols such as Pseudomonas fluorescens and Trichoderma viride (Deepthi and Reddy, 2014).
Sclerotinia Stem Rot
Aetiology
Sclerotinia sclerotiorum Lib. de Bary is the incident of stem rot or cottony stem rot in the chrysanthemum.
Symptoms
Sclerotinia stem rot refers to the clusters of white cottony fungal mycelium, mainly produced under humid conditions that commonly sprout from the blight or lesions. The disease produces stem rot or blight and/or head rot in chrysanthemums (Agrios, 2005). When plants are first attacked at their bases, the first sign is a dark green moist rot at the base of the stem. Rapid girdling and death of the stem result in the rapid wilting and eventual demise of the plant.
Biology and Epidemiology
Sclerotinia may also survive as saprophyte in the absence of host, which allows it to persist on a wide variety of plant species. They primarily survive through sclerotia, while ascospores and mycelium in diseased tissues have a much lower chance of survival (Cook et al., 1975; Willets and Wong, 1980). Sclerotia subjected to low temperatures result in the development of apothecia, which produce ascospores (tiny, pinkish to buff saucer-shaped structures). For 2-3 weeks, apothecia develop in damp environments and then forcefully release ascospores into the air, spreading the disease on the above-ground parts (Abawi and Grogan, 1975; Agrios, 2005). Once the fungal hyphae invade, the host cells are quickly destroyed.
Management
The diseased plants must be promptly removed and burnt. Greenhouse benches and floors should be cleaned using a product approved for greenhouses; pots or transplant trays in which the disease has been identified should be discarded. When Sclerotinia is detected in a greenhouse, thiophanate-methyl should be sprayed on the plants. Against the sclerotia of Sclerotinia spp., a biocontrol fungus Paraconiothyrium minitans (=Coniothyrium minitans) should be used (Ojaghian, 2009).
White Rust
Aetiology
Chrysanthemum white rust is caused by Puccinia horiana Henn.
Symptoms
Tiny blisters of the size of pinheads develop undersides of the leaves. There is also some evidence of blistering on the upper surface. The spores form a dark brown powder and may be seen when the blisters burst.
Biology and Epidemiology
Thirteen distinct chrysanthemum species and the closely related genera Leucanthemella and Nipponanthemum are susceptible to white rust. Puccinia horiana produces teliospores and basidiospores, depending on the stage of development. Unless the pustules are scrubbed vigorously, they will develop into teliospores. Basidiospores, which are produced by the teliospores, are the “infectors” and may spread rapidly under susceptible conditions, leading to an epidemic. Splashing water and human handling are the main vectors for their dissemination from plant to plant. In order to spread infection, basidiospores need to be coated in a thin layer of water. These spores are the major inoculum in infectious disease epidemics. At temperatures between 16 and 27 °C, infections might spread.
Management
It is best to get the cuttings from a commercial nursery. It is important to minimise moisture around plants and get rid of dead plant matter. Preventative fungicide spraying should be done every 7-10 days (McCain and Gonot, 1979). Both brown rust and white rust may be effectively treated using preventative and curative fungicides.
Brown Rust
Aetiology
Chrysanthemum brown rust is caused by Puccinia chrysanthemi and P. tanaceti. Brown rust causes yellow spots on both leaf sides, which eventually turn into powdery, dark brown pustules that are more often observed on the leaf undersides. The primary pustule frequently darkens to a brown or black colour, and it is surrounded by concentric rings of subsequent pustules. Differentiating brown rust from white rust is as simple as looking for uredospores and seeing that they are a different colour than the pustules and spores. Dark, rust-coloured teliospores are found in brown rust (Peterson et al., 1978). For management keep the soil warm or treat it with chemicals on a regular basis. Soil solarization greatly reduces disease prevalence.
Botrytis Blight
Aetiology
Botrytis cinerea Pers.: Fr. Botrytis is the causative agent of botrytis blight in chrysanthemum. This is a frequently occurring and difficult-to-control disease in chrysanthemum cultivation.
Symptoms
Fungus may infect all above-ground tissues, but it does the most economic damage to flower structures. The pathogen attacks flowers in moist greenhouses causing the development of water-soaked brown lesions, which later become covered with the mass of fungal spores greyish-brown in colour.
Biology and Epidemiology
This fungus needs high moisture, and spore development requires a period of leaf wetness. This fungus may act as either a pathogen or a saprophyte, and as a result, it can live on many species of plants (Daughtrey et al., 1995). The fungus survives as mycelium (Yunis and Elad, 1989) and sclerotia (Ellis and Waller, 1974), and the conidia spread through air currents and water splash. To infect a host, the conidia of B. cineria must remain moist for 5-8 hours (Jewett and Jarvis, 2001). Periods of dryness (desiccation) can destroy the spores during germination.
Management
Increasing air flow by planting the plant at the recommended spacing. Bavistin (0.1%) and Copper oxychloride (0.2%) sprays are effective. It is possible to control Botrytis with a variety of biorational (bicarbonates and polyoxin D) and biological (Bacillus subtilis, etc.) products.
Root Rot
Aetiology
Pythium aphanidermatum (Edson) Fitsp; P. carolinianum Matthews, P. debaryanum Hesse, P. helicoides Drechsler, P. dissotocum Drechsler; P. oedochilum Drechsler; P. sylvaticum Campbell et Hendrix; P. ultimum Trow var. ultimum, and also some other Pythium spp. have been responsible for causing root rot in chrysanthemum which is also called a basal stem rot.
Symptoms
The most typical symptoms on chrysanthemums are stunting and the onset of pythium root rot and basal stem rot. Aerial symptoms of infection, which manifest on cuttings or on immature plants, can also develop. Symptoms are highly reliant on high relative humidity and soil moisture. Pythium root rot may be differentiated from other root rots by its waterlogged, sloughed roots and dark brown necrotic stem with a damp look. Wilt is sometimes the sole visible symptom. Foliar symptoms are necrotic as well, giving the impression of being damp rather than dry. Pythium mycelium can be formed on leaves in very damp and humid conditions, although it is easily removed by mechanical means such as human touch or cutting instruments.
Biology and Epidemiology
Like Phytophthora, Pythium is a soilborne pathogen that may remains latent in the soil for long periods of time in the form of chlamydospores or oospores, which are typically associated with plant detritus. Hyphae and zoospores are also infective units, although their lifespans are typically shorter. The spores of pythium may live for a long time in the soil, as well as in the soil of transplant trays and containers. Pythium most commonly spreads in greenhouses by the introduction of contaminated plant materials, hands, equipments and hose nozzles. Abundant soil moisture in both greenhouse and outdoor settings is favourable to disease growth, and water splashing causes severe dissemination.
Management
The disease is exacerbated by cultural factors. Sound horticultural techniques are essential in avoiding this disease since stressed plants are more vulnerable to infection. To reduce compaction, it is best to avoid lengthy periods of saturated media and to make use of a medium that provides excellent aeration. Keep an eye on the roots, and try to keep them from being too dry or exposed to too much soluble salts. Whenever possible, clean water and sanitation should be provided. Fenamidone, pyraclostrobin, etridiazole, cyazofomid, propamocarb, fosetyl-Al, and phosphorous acid are effective fungicides that can be used to manage Pythium infection.
Fasciation
Aetiology
Bacterial Fasciation or leafy gall of chrysanthemum is caused by the bacteria, Rhodococcus fascians (syn. Corynebacterium fascians).
Symptoms
The bacteria causes stunting of growth and an abnormally high number of buds in florists by shortening and thickening the stem towards the base and causing the production of aborted and malformed leaves (Pirone, 1978). More fasciation symptoms appear with longer infected period of times (Oduro, 1975).
Biology and Epidemiology
There are more than hundred different flowering plants, vegetable varieties, and fruit trees that are infected by Rhodococcus fascians (Goethals et al., 2001; Putnam and Miller, 2007). When temperatures and humidity are high, the disease becomes more severe. Infected plant parts, tainted soil (Faivre-Amiot, 1967), and contaminated seeds are important sources of bacterial survival and spread (Pataky, 1991), however, R. fascians spreads mostly through water (Putnam and Miller, 2007).
Management
Any potential pathogens in the seed stock must be removed. Disease outbreaks are common in raising plant production units without good cleanliness practics. Infected plants should be rogue out, freshly acquired stock plants should be separated from infected plants. Monitoring the plants for signs of disease, if diseased stock plants should be renewed, and surfaces, containers, and growth medium are disinfected and sterilised (Putnam and Miller, 2007). Steam at 82.2 oC for 30 min or 71.1 oC for 1 h can disinfect soil (Pataky, 1991).
Viral Disease
Various viruses from the family Bunyaviridae cause diseases in chrysanthemum. Some of the most notable viruses include the chrysanthemum stem necrosis virus (CSNV), Impatiens necrotic spot virus (INSV), iris yellow spot virus (IYSV), and Tomato spotted wilt virus (TSWV).
Symptoms
Depending on the chrysanthemum variety and growing conditions, symptoms might include chlorotic leaf spots, necrosis of the leaves and axillary shoots, black streaks on the stem, wilting, the loss of the apical bud, mortality to young plants etc. Leaves become distorted and chlorotic with necrotic patches, rings on leaves, and necrotic streaks on stems.
