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

FUNGAL DISEASES (PART – I)

1. ROOT ROT AND WEB BLIGHT

Disease is of economic importance especially when crop is grown during rainy season and is more prevalent on bush type than pole type bean.

Symptoms: The symptoms of the disease appear in following two phases:

Root rot:

• Symptoms appear on roots and stem above and below the surface of the soil as reddish brown, sunken cankers (Plate-1).
• The lesions enlarge rapidly and girdle the stem at the collar region, extending longitudinally downward to the roots leading to partial or complete rotting of the root system.

Web blight:

 

• The symptoms on leaves appear as small, circular, water soaked spots which later become tan to brown in colour (Plate-2a) and these spots coalesce to form larger areas on the leaf blade.
• The pods are attacked at all stages of their growth and dark brown, more or less circular, slightly zonate and definitely sunken spots appear on pods which in moist weather are covered with hyphae and sclerotia of the fungus (Plate 2b).
• Tan brown to reddish brown discoloration is observed on infected seed.

 

Pathogen:

• The disease is caused by Rhizoctonia solani Kuhn.
• The basidial stage of the fungus is Thanatephorus cucumeris (Frank) Donk but occurs very rarely.
• The fungus consists of septate mycelium, the mycelium is branched near the distal septum of the mother hyphal cells at right or acute angles and the branches are constricted at or near the point of origin of the septum.
• In culture, sclerotia develop within 5-6 days.

Disease cycle and epidemiology:

• The pathogen perpetuates either through infected seed or in soil as sclerotia (Fig. 1.).
• The infected seeds after germination either rot in the soil itself or give rise to plants which later get damped off.
• The rain splashes spread the soil borne inoculum to foliage that leads to web blight symptoms.
• High rainfall and soil moisture coupled with high relative humidity and soil temperature of 23 to 25oC favour disease development.

Management:

• Collect and destroy the infected plant debris.
• Follow long crop rotation.
• Solarize field soil at least for 40-45 days during summer months.
• Amend the soil with neem or mustard cake (250g/m2).
• Use healthy seed.
• Dry seed treatment with carbendazim (0.2%).
• Before the onset of monsoon rains, apply pine needle/ grass mulch on the field floor to avoid splashing of pathogen inoculum to foliage.
• Spray the crop with carbendazim (0.1%) or mancozeb (0.25%) or combination of mancozeb (0.25%) and carbendazim (0.05%) or tebuconazole (0.06%) and repeat at 10-14 days interval for foliar control of the disease.

2. ANTHRACNOSE

The disease is more severe in temperate and subtropical mountaneous regions of the world with cool and wet climate.

Symptoms:

• Most striking symptoms of the disease appear on immature pods as brown and sunken spots with lighter or grey central area (Plate-3). The central portion of the spots shows pinkish masses of fungal spores, especially in wet weather.
• Later, the sides of these spots appear raised.
• On leaves, the infection appear on the under leaf surface as blackened dead portions of the veins which may extend to limited adjoining areas.
• In severely infected pods, brown to light chocolate coloured sunken cankers may also develop on seed coat.

Pathogen:

• The disease is caused by Colletotrichum lindemuthianum (Sacc. and Magn.) Briosi and Cav.
• The mycelium is branched, septate and hyaline at first becoming dark with age.
• The acervulus contains a layer of 3 to 50 conidiophores depending on the size of lesion (Plate-4).
• Conidia are borne successively and acrogenously on short conidiophores under favourable conditions until pinkish spore masses appear on the surface and setae are produced sparingly.
• Conidia are hyaline, cylindrical having rounded ends or somewhat pointed at one end and often bear a clear vacuole like body near the centre.
• The teliomorph of the pathogen has been identified as Glomerella cingulata (Ston.) Splaud & Schrenk (=G. lindemuthiana Shear).
• Perithecia contain hyaline and filiform periphyses and asci.
• Each ascus contains eight allantoid ascospores, which are ejected from the tip of ascus.

Disease cycle and epidemiology:

• The pathogen perpetuate in infected seeds and in infected plant debris (Fig.2).
• When infected seed germinates, lesions appearing on cotyledons which serve as the source of secondary inoculum producing the spores as water borne.
• Primary leaves and the hypocotyl are foci of secondary infections.
• The teliomorph of the fungus is rarely found in nature.
• It develops most abundantly in cool, wet weather and largely disappears under hot and dry conditions.
• A relative humidity of 92 per cent and above is essential for infection, the optimum being close to 100 per cent.
• The fungus requires about 10 mm of rain to establish initial infection.
• The optimum temperature for disease development ranges from 18 to 27^oC with maximum intensity at 21^oC and is markedly reduced at 13^oC.
• The movement of insects, animals and man may spread conidia particularly when foliage is moist.

Management:

• Follow 2-3 years of crop rotation with non-leguminous crops.
• Use disease free seed and treat them with carbendazim or benomyl (0.2%).
• Resistant cutivars/ lines like KRC 1, KRC 17, EC 42960 and EC 57080 have been identified as resistant to this pathogen.
• In field, the spread of the disease can be limited by application of fungicides like mancozeb (0.25%) or combination of mancozeb (0.25%) and carbendazim (0.05%) or tricyclazole (0.03%) and repeat at 10 -14 days interval.

3. ANGULAR LEAF SPOT

The disease was first reported by Saccardo (1878) from Italy. In India disease was first noticed from Nilgiri hills by Srinivasan whereas from H.P. by Sohi et al. (1963).

Symptoms:

• The symptoms appear as circular spots on cotyledonary leaves while, 3-5 angled spots appear in between veins and veinlets on true leaves which are dark grayish in colour on upper surface while light gray on the lower surface (Plate-5a).
• With the passage of time, spots change to reddish brown and finally to dark brown colour and on close observation, the spots reveal the presence of coremia bearing large number of spores.
• In severe infections, leaves show upward curling and defoliate prematurely.
• The fungus also attacks the pods causing superficial, smooth, usually circular spots with reddish brown center and ashy black borders but with time deeper tissues are also involved (Plate 5b).
• Elongated dark brown lesions also appear on stem and petioles.
• Under severe conditions of infections, these spots coalesce causing complete defoliation.
• Yellowish brown discolouration is also observed on infected seeds located underneath the pod lesions.

Pathogen:

• The fungus responsible for this disease is Phaeoisariopsis griseola (Sacc.) Ferr. (Syn. Isariopsis griseola).
• The fungus produces a stromatic structure in the substomatal cavity.
• Columnar hyphae producing conidiophores are laterally joined together in the form of synnemata or coremia, which are darker at the base and lighter towards the apex and either curved, flexuous or septate (Plate-6).
• Conidia are light grey to hyaline, cylindrical to spindle shaped and straight to slightly curved, 1-3 (rarely 4-6) septate which are borne singly on the tips of conidiophore.
• No sexual stage of the fungus has yet been observed.

Disease cycle and epidemiology:

• The pathogen perpetuate both on seed and in infected plant debris (Fig.-3).
• Infected seed germinates to give rise to infected plantlets bearing coremia of the fungus on the undersurface of the cotyledonary leaves.
• The conidia released from the coremia disseminate either by rain splashes or with wind blown soil particles.
• The conidia germinate to form germ-tube, which enters through stomata and advances intercellularly in the mesophyll and palisade parenchyma.
• Within 7-10 days of infection, the fungus produces necrotic lesions, develops stromata in the substomatal cavity, which may sporulate during 24 to 48 h of continuous moisture.
• The conidia continue the cycle by acting as secondary inoculum.
• Infection from pods also goes to the developing seed.
• Optimum temperature for growth and sporulation, spore germination and development of the disease are 21-24°C, 18-24°C and 24°C, respectively while, optimum RH for sporulation, spore germination and symptoms development is >90 per cent.

Management:

• Collect and destroy the infected plant debris.
• Follow at least 2-3 years crop rotation with non-leguminous crops.
• Use healthy seed and treat them with carbendazim (0.2%).
• With the initiation of the disease, spray the crop with mancozeb (0.25%) or combination of mancozeb (0.25%) and carbendazim/ benomyl/ thiophanate methyl (0.05%) or hexaconazole (0.05%) and repeat at 10-14 days interval.