1. Phomopsis blight
The plants are attacked at all stages of growth.
- In nursery bed, it causes damping off which results from the infection of the seedlings at the collar portion just above the soil line. After transplanting, circular grey spots with light coloured centers appear on the leaves (Plate 1a).
- In later stages, the lighter portion is studded with numerous black pycnidia.
- Affected leaves turn yellow and fall down pre-maturely.
- At the base of the stem, the fungus causes characteristic constrictions leading to canker development and toppling of the plants.
- On fruits, the disease manifests as pale sunken spots, which later enlarge and cover the entire fruit surface. A large number of dot like pycnidia also develop on such spots (Plate-1b). If infection of fruits takes place through calyx, the whole fruit becomes mummified due to dry rot.
- The fungus responsible for this disease is Phomopsis vexans (Sacc. and Syd.) Harter. Diaporthe vexans Gratz is the perfect stage.
- The mycelium is hyaline and septate.
- The conidiophores (phialides) in the pycnidium are hyaline, simple or branched, sometimes septate and arise from the innermost layer of cells lining the pycnidial cavity.
- The pathogen is reported to produce two types of conidia viz., alpha and beta in its pycnidium.
- Formation of conidia in pycnidia of P. vexans is temperature dependent. At low temperature (10-16oC), the pathogen produces beta conidia and at high temp. (25-28o C), the alpha conidia.
- These two forms of conidia get inter-converted when subjected to specific temperature and alpha and beta are two forms of the same conidium.
- P. vexans produces only one type of conidia in its pycnidia during summer months. Conidia are hyaline, one celled and sub cylindrical which gradually changed into beta form.
- Beta form of conidia, the stylospores is filiform, curved, hyaline and septate.
- These spores normally do not germinate. But inoculation of host with beta conidia caused interveinal necrosis.
- Perithecia are observed only in cultures which are usually in clustures, beaked, carbonaceous, sinuate, and with irregular ostiole.
- Asci are clavate, sessile and contain eight ascospores.
- Ascospores are hyaline, narrowly ellipsoid to bluntly fusoid, one-celled, septate and constricted at the septum.
Disease cycle and epidemiology:
- The pathogen is seed borne and also survives in plant debris both as mycelium and pycnidia (Fig.1.).
- Seed infections directly lead to diseased seedlings.
- The pycnidiospores are disseminated through rain splashes, irrigation water, agricultural implements and insects.
- High relative humidity coupled with high temperatures is favourable for disease development.
- Pathogen grows best at a temperature of 21 – 32.5°C with a optimum temperature of 28 – 29°C. Storage rot is maximum at a temperature of 25°C.
- Maximum disease development takes place at about 26oC under wet weather conditions.
- Cultural practices like collection and destruction of the diseased plant debris, crop rotation and use of disease free seeds are recommended to reduce the initial inoculum.
- Treat the seed with carbendazim (0.2%) or thiophenate methyl (0.2%). With the initiation of the disease, spray the crop with carbendazim (0.1%) or combination of mancozeb (0.25%) and carbendazim (0.05%) or copper oxychloride (0.3%) and repeat at 10 to 14 days interval.