Diseases of Vegetable, Ornamental and Spice Crops

FUNGAL DISEASES (PART – II)

4. WHITE ROT
Symptoms:

• The symptoms of the disease appear as water-soaked lesions, which may develop on any part of the plant, but occurs mainly on the stems or branches (Plate-5).
• At the point of infection, a dry, discoloured spot develops.
• Under cool and humid conditions, the mycelium emerges and can be seen sticking to affected portions.
• Pods are also infected and flesh start rotting and in the rotting tissues, a large number of sclerotia of the fungus can be seen.
• The diseased tissues become whitish and may be shredded.
• The seeds also become shriveled and discoloured. Sclerotia appear on affected pods also.

Pathogen:

• The disease is caused by Sclerotinia sclerotiorum (Lib.) de Bary. The pathogen has been described in detail in the lecture of cauliflower diseases.

Disease cycle and epidemiology:

• The pathogen mainly perpetuates through sclerotia for many years in soil.
• It is also same as in cauliflower diseases lecture.
• Sclerotia are primary source of infection and the fungus is polyphagus in nature.
• Optimum temperature for initiation of the disease is 15.5 – 21^o C and free moisture for 42-75 h is essential for establishment of infection and lesion expression.

Management:

• Collect and destroy the infected plant debris.
• Follow crop rotation with paddy or maize.
• Maintain wider row spacing.
• Use resistant sources if available.
• Apply Trichoderma harzianum formulation (120 kg/ ha) at the time of sowing.
• Apply foliar sprays of carbendazim (0.1%) at flowering stage and repeat at 10-14 days interval.

5. ASCOCHYTA BLIGHT OR FOOT ROT

Three morphologically distinct species of Ascochyta are reported to be associated with this disease which causes different types of symptoms including leaf and pod spot, blight and fruit rot.

Symptoms:

Ascochyta or Mycosphaerella blight:

• This disease is characterized by brown to purplish, irregular areas on the foliage which later become circular and somewhat zonated.
• The small, brown to purplish irregular spots, which appear on the pods, enlarge to irregular purplish area or large area could become blotched with the coalescing of lesions (Plate-6).
• Developing seeds may be deformed, remain small in size and are sometimes stained.
• Black to purplish streaks appears on stem.
• In lesions, pycnidia appear in characteristic ring pattern.

Ascochyta foot rot:

• The symptoms are similar to those described for blight but in this case foot rot lesions predominate (plate-7).
• The dark brown lesions on the stem start soon after germination below the soil line and gradually extend down to roots and up the stem.

Ascochyta leaf and pod spot:

• Symptoms of this disease appear as definite, circular, sunken, tan to brown spots on leaves and pods while elongated on stems and petioles (Plate-8).
• A dark brown margin usually surrounds the tan spot and pycnidia may form on these spots also.
• Underlying seeds in the pods may also be affected and remain shriveled.

 

Pathogen (s): Different Ascochyta species causes following diseases:

Blight Ascochyta pinodes (Berk & Blox) Vestgrn. (perfect stage Mycosphaerella pinodes (Berk & Blox.) Vestergr),
Leaf and pod spot Ascochyta pisi Lib.
Foot rot Phoma medicaginis var. pinodella (Jones) Boerema (syn. Ascochyta pinodella Jones.).

Ascochyta pinodes:

• Perithecia are globose having beaked ostioles and contain bitunicate, cylindrical, clavate asci.
• Each ascus contains eight ascospores which are bicelled and hyaline.
• The fungus colonies are light to dark gray in colour and often have concentric rings.
• The conida have large number of globules.
• The spore ooze collectively appears buff coloured.

Ascochyta pinodella:

• It produces single sometimes bicelled hyaline spores in pycnidia.
• This species is not known to produce perfect stage.
• In culture, fungus produces numerous chlamydospores singly or in chains.
• Colonies are dark gray and turn black at maturity.

Ascochyta pisi:

 

• Conidia are longer and thin with slight constriction at septation.
• Colonies are light in colour and exude spore mass which is carrot red in colour.

 

Disease cycle and epidemiology:

• Different Ascochyta species perpetuates either in the seed or in diseased crop debris.
• In crop debris, these species survive as perithecia, sclerotia or chlamydospores.
• The pathogens may also survive through collateral hosts which are Lathyrus sp. and Vicia spp., for A. pisi; Phaseolus and Lathyrus for A. pinodes and Vicia, Trifolium and other legumes for Phoma medicaginis var. pinodella).
• Cool weather coupled with presence of sufficient moisture on the leaves is the most favourable conditions for disease development.
• The disease attains serious proportions during prolonged rainy periods.
• Atmospheric humidity >90 per cent helps in increasing the disease incidence while no infection occurs below 80 per cent RH.

Management:

• Use disease free seeds.
• Disease free seeds can only be produced if the crop is raised in low rainfall areas.
• The soil borne inoculum can be reduced by following long crop rotations and by destruction of the crop refuse by burning either in the field or after threshing.
• Use resistant sources, if available.
• Seed dressing with microbial cultures of Trichoderma koningii and Gliocladium roseum (4 g/kg seed ) was also found effective in protecting the crop against A. pisi.
• Treat the seed with fungicides namely carbendazim (0.2% ), benomyl (0.1%), thiabendazole (0.1% ) + thiram (0.3% ) and captan (0.3% ).
• With the initiation of the disease spray the crop with fungicides like chlorothalonil (0.2%), carbendazim (0.05%) or combination of both and repeat at 10 days interval.