Diseases of Vegetable, Ornamental and Spice Crops (2+1)

FUNGAL DISEASES

1) Downy mildew:
Symptoms:

• Pale green to yellow angular spots restricted by leaf veins appear on the leaves (Plate-1).
• Corresponding underside of these spots is covered by grey to brownish growth of fungus.
• Later on, lesions turn necrotic.

Pathogen:

• The disease is caused by Pseudoperonospora cubensis (Berk. & Curt.) Rostow.
• The mycelium is hyaline, coenocytic and intercellular.
• Sporangiophores develop in groups of 1-5 and are bulbous at base and branched dichotomously.
• Sporangia form singly on subacutely shaped sporuliferous tips, are pale grey to purple in colour, ovoid to elliptical in shape, possess thin wall with a papilla at distal end.
• Sporangia germinate by producing zoospores.
• Oospores are produced rarely and are spherical, rarely obovoid to ellipsoid ,light yellow to pale yellow with smooth wall.

Disease cycle and epidemiology:

• The pathogen mainly survives as mycelium and sporangiophores in living hosts during off-season.
• Survival through oospore is not common.
• Moderate temperature (20-22^o C) coupled with high rainfall and > 80 per cent RH favour disease development and spread.

Management:

• Destruction of plant debris of previous crop.
• Destruction of cucurbitaceous weeds from in and around the field.
• Maintain proper plant density.
• Spray crop with mancozeb (0.25%) followed by one spray of metalaxyl + mancozeb or cymoxanil + mancozeb (0.25%) and two sprays of mancozeb (0.25%) at 10 days interval.

2. Powdery Mildew
Symptoms:

• Disease appear as white, floury coating on the leaves, stems and other succulent parts (Plate-2).
• In S. fuliginea, the spots are dirty brown in colour while in E. cichoracearum white in colour.

Pathogen:

• Two fungi namely Sphaerotheca fuliginea (Schlecht.ex. Fr.) Poll. and Erysiphe cichoracearum DC. are associated with this disease.
• Conidia of S. fuliginea are formed in long chains and often show fibrosin bodies , are ellipsoidal shaped while in E. cichoracearum conidia are single celled, hyaline and barrel to cylindrical and are in long chains.
• Cleistothecia of S. fuliginea contain single ascus with eight ascospores while cleistothecia of E. cichoracearum contain 8-18 asci with 2-3 ascospores.

Disease cycle and epidemiology:

• The pathogens overwinter either in the form of cleistothecia or conidia in cucurbitaceous weeds in the neighbouring areas.
• Moderate to warm temperature (25 ⁰ C) coupled with high relative humidity and reduced sunshine hours favour the disease development.

Management:

• Collect and burn the infected plant debris.
• Spray the crop with fungicides like dinocap (0.06%), carbendazim (0.05%) or hexaconazole (0.05%) or difenoconazole (0.03%) and repeat at 10-14 days interva.
• Precaution: Do not spray sulphur fungicides as cucurbits are sensitive to these.

3) Anthracnose
Symptoms:

• Rough, circular, light brown to reddish brown lesions appear on leaves (Plate-3a).
• On fruits, roughly circular, sunken, water-soaked spots with dark borders (Plate-3b) appear.
• Old spots turn black and are covered with pink spore masses under moist weather.

Pathogen:

• The disease is caused by Colletotrichum orbiculare (Berk. & Mont.) Arx. (Perfect stage: Glomerella lagenarium F. Stevens)
• The fungus produces black stromata, bearing black setae and hyaline conidiophores.
• Conidia are produced on conidiophores by budding and are one celled, hyaline, oblong to ovate.

Disease cycle and epidemiology:

• The fungus is both soil and seed borne.
• Cucurbitaceous weed hosts serve as source of perennation.
• Conidia are disseminated by moist wind or rain splashes or carried through implements.
• Spore germination is optimum at 22-27^o C temperature and 100 per cent RH.

Management:

• Deep ploughing of crop residues immediately after harvest.
• Use healthy seed and treat seed with carbendazim (0.2%).
• Spray the crop with carbendazim (0.1%) or mancozeb (0.25%) or their combination at 10-14 days interval.

4) Fusarium wilt
Symptoms:

• Initial symptoms appear as yellowing and marginal necrosis of leaves from down to upwards.
• Sudden drooping and wilting of plants.
• Vascular discolouration may or not be present.

Pathogen:

• The disease is incited by Fusarium oxysporum Schlechtend ex. Fr. f.sp. cucumerinum Owen.
• The fungus produces septate mycelium and micro and macro conidia.
• Chlamydospores are also produced.

Disease cycle and epidemiology:

• The fungus is soil borne and survives as chlamydospores.
• A temperature between 18-22^o C, high light intensity and relative humidity favour the disease development.
• High nitrogen and low potassium enhance disease incidence.

Management:

• Follow crop rotation with non cucurbitaceous crops.
• Soil solarization for 40-45 days during summer months.
• Apply soil amendments like margosa and mustard cake (250 g/m2).
• Application of non-pathogenic isolates of Fusarium or Pseudomonas putida as bioagents also help in the management of this disease.
• Drench the plants with carbendazim (0.1%).