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

FUNGAL DISEASES (PART – II)

4. FLOURY LEAF SPOT
Symptoms:

• The disease appears in the form of white tufty growth of the fungus on the lower leaf surface appearing as sprinkled with coarse flour, hence the disease is named as floury leaf spot (Plate-6).
• The corresponding upper side of the leaf turns yellow, then brown and finally dry up. Young leaves are less susceptible.

Pathogen:

• The pathogen responsible for this disease is Mycovellosiella phaseoli (Drummond) Deighton (Syn.: Ramularia phaseolina Petrak).
• It is an obligate parasite.
• On the under surface of host, colourless conidiophores emerge in tufts from the stomata and interwine in clusters around the leaf hairs.
• Erect, more or less divergent, somewhat tapering towards the tip, simple or irregularly dichotomous, a few short, erect, single conidiophores were also found scattered over the leaf surface.
• Conidia are variable in shape and size (5.24-21.4 x 3.5- 5.3 µm) mostly ellipsoid or ovoid, tips acute or obtuse, base papillate at the point of attachment, stright hyaline, commonly aseptate, rarely one septate.

Disease cycle and epidemiology:

• The fungus is not seed borne and perpetuate in infected plant debris as conidia from one season to another.
• Overwintering conidia may be splashed by rain to lower leaves, which in the presence of free water germinate on host surface and cause infection.
• Conidia produced on primary spots are blown by wind and cause secondary infections.
• The disease is more common in altitudes of 1500 to 2000 m with heavy rainfall. The optimum temperature for conidial germination and germ tube elongation is 20^oC.

Management:

• Collect and destroy the infected plant debris.
• Follow at least three years of crop rotation with non-leguminous crops.
• Spray the crop with carbendazim/ benomyl (0.1%) and repeat at 10-14 days interval.

5. CHARCOAL ROT OR DRY ROOT ROT

The disease is also known as ashy stem blight, root rot, ashy grey stem and Macrophomina rot.

Symptoms:

• The initial symptom is yellowing of leaves, which soon dry up.
• The plant may wilt within a week after the appearance of symptoms.
• When the stem is examined closely, dark lesions may be seen on the bark at the ground level.
• On uprooting infected plants, dry rot symptoms may be seen on the basal stem and the main roots.
• The tissues are weakened and break off easily.
• Black sclerotial bodies are formed on the diseased tissue a few days after infection.

Pathogen:

• The disease is caused by Macrophomina phaseolina (Tassi) Goid.
• The sclerotial stage of the fungus is known as Rhizoctonia bataticola (Taub.) Butler.
• The mycelium is superficial or immersed, hyaline to brown, branched, septate and often tree-like in form.
• The pycnidia are at first buried beneath the lead-grey epidermis in which many sclerotial bodies are generally mixed.
• The conidiophores are more or less straight, sometimes crooked with a truncate tip.
• The conidia are one celled, more or less fusiform, straight or slightly curved.
• One end of the conidium is often pointed and the other is blunt.
• Sclerotia are readily produced, both on the plants in the field and in cultures.
• These are jet black, smooth, hard round to oblong or irregular in shape.

Disease cycle and epidemiology:

• The pathogen perpetuates as sclerotia in the soil and in infected seed.
• Survival is longer in dead host tissues than in free conditions in the soil.
• The fungus is a poor competitor in soil but readily colonizes plant debris.
• Population of the fungus increases when susceptible hosts are continuously grown in the field.
• When infected seed is sown in the field, it also introduces the fungus in the field.
• The sclerotia germinate and the mycelium cause primary infections.
• Environmental factors contributing to soil moisture stress/ water deficit predispose the plant to infection. Soil temperature of 28^oC or more favours disease development.

Management:

• Collect and destroy the infected plant debris.
• Follow long crop rotations with non-leguminous crops.
• Solarization of infected soils during summer months can also help in reduction of soil borne inoculum.
• Use disease free seed and treat them with carbendazim or benomyl (0.2%).
• Application of biocontrol agents like Trichoderma viride and Aspergillus niger have also been found promising but need further confirmation.

6. RUST
Symptoms:

• On leaves, the disease may appear on both the surfaces, but is more common on the abaxial surface (Plate-7).
• Pustules of the disease usually appear first as small and slightly raised spots that are almost white in colour.
• These pustules enlarge by forming reddish brown sori, up to 2 mm in diameter, containing the urediniospores.
• After coalescing, they may occupy larger areas.
• A ring of secondary sori may develop around the original infection on susceptible cultivars. Telial stage is linear, dark brown to black colour.
• The affected leaves may turn yellow and dry or fall prematurely.

Pathogen:

• The disease is caused by Uromyces phaseoli typica Arth. and U. appendiculalus (Pers.) Unger. Being autoecious rust, all its spore stages are produced on bean plant but pycnial and aecial stages are not commonly encountered in nature.
• The urediniospores are brownish, globoid or ellipsoid, one celled and echinulate.
• Spore wall is golden brown 1-1.5 µm thick having two equatorial pores (Plate-8).
• Teliospores are one celled, pedicellate, globoid or broadly ellipsoid.
• They may be smooth or possessing a few verucose marks on the wall.
• The teliospores wall is chestnut brown in colour with a hyaline papilla over the pore.

Disease cycle and epidemiology:

• In areas where the crop is grown continuously and collateral hosts are also present, inoculum in the form of urediniospores is available round the year.
• The life cycle of the fungus is completed on the host itself; however, in cooler areas where bean is taken as single crop in the year, teleutospores surviving in crop debris serve as source of perpetuation.
• Areas having high humidity and temperature upto 24^o C are most suitable for disease development.
• Urediniospores germinate optimally at 15-24^o C while for teliospores germination, 10-15^o C is optimum.

Management:

• Collect and destroy the infected plant debris.
• Follow long crop rotations.
• With the initiation of the disease, spray the crop with mancozeb (0.25%) or hexaconazole (0.1%) or difenoconazole (0.05%) or combination of mancozeb (0.25%) and hexaconazole (0.05%) and repeat at 10 days interval.