| 1. FUSARIUM WILT
- The disease can appear on any stage of plant growth and the crops planted during summer months suffer more than the crops planted during February-March.
- The characteristic symptoms of the disease appear as yellowing and stunting of the plants followed by wilting and rolling of the leaves (Plate-1) and finally the plant dies.
- Before the appearance of typical wilting symptoms, the leaves hang down during daytime and recover again in the night but ultimately they wilt and die.
- Vascular bundles of the affected plants appear as dark streaks and the whole stem is blackened in case of severe infection.
Disease cycle and epidemiology:
- The disease is caused by Fusarium oxysporum f.sp. vasinfectum (Atkinson) Snyder and Hansen.
- The mycelium is hyaline and intracellular in the host.
- Macroconidia are mostly 3 septate and microconidia are 0 to 1 septate.
- Macroconidia are fusiform, falcate, curved and formed on sporodochia and pionnotes. In mass these conidia appear buff to salmon orange in colour.
- The microconidia are 5-12 x 2-3.5 µm while macroconidia are 40-50 x 3-4.5 µm in size.
- Both intercalary and terminal chlamydospores are formed which are broadly ovate.
- The fungus survives from one season to other in the form of chlamydospores and in infected seeds.
- In contact with the host roots, the chlamydospores or conidia germinate and penetrate.
- The pathogen proliferates extensively in the cortical tissues, extending up and down.
- After some growth in root cortex, the pathogen reaches the xylem vessels where it multiplies very rapidly and result in browning of xylem vessels and clogged at further places.
- The optimum temperature for growth of the fungus is 25oC and for disease development ranges between 22 – 28o C.
- Moisture is not that much important for infection.
- Cultural practices like long crop rotations, exposing the soil to the sun during summer months by deep ploughing, soil solarization and destruction of diseased roots are some of the practices which can reduce the disease.
- Some cvs. of okra like Pusa Sawani and Pusa Makhmali have been reported resistant to this disease.
- Use healthy seed and treat with carbendazim (0.2%).
2. CERCOSPORA LEAF SPOT
- Three different species of Cercospora are associated with this disease. The symptoms of each species are described below:
- The fungus produces indefinite leaf spots but grows as a sooty mould on the lower surface of the leaves (Plate-2).
- Severely affected leaves roll, wilt and fall down to the ground.
Cercospora hibiscina: The fungus produces dark olivaceous patches of mould on the lower surface of the leaves.
Cercospora malayensis: The spots caused by this species produce definite leaf spots with grey centers and red to purple borders.
Disease cycle and epidemiology:
- The disease is caused by three species of Cercospora like C. abelmoschi Ell. & Ev., C. hibiscina Ell. & Ev. and C. malayensis Stevens.
- These species differ in their size of conidiophores and conidia.
- In C. abelmoschi, the conidiophores are long, brown and bear pale olivaceous, slightly tapered conidia.
- The conidiophores of C. hibiscina are extremely long sometimes up to 1000 µm in size, narrow and bear conidia that are sometimes hyaline and appreciably more narrow then those of C. abelmoschi.
- The conidiophores of C. malayensis are borne in clusters of 5-20 and bear conidia, which are colourless, narrow, long, and tapering from the blunt base to the sharp tip.
- The fungus overwinters in the infected plant debris as conidia or stromata in soil and also on the wild species of Abelmoschus.
- The conidia in favourable weather conditions, germinate and cause infection of the host through stomata.
- The spores produced on the primary spots are blown by wind and cause secondary infections.
- The three species cause infection in moderate temperatures (25-29o C) and high humidity.
- Cultural practices like collection and destruction of infected plant debris, crop rotation and destruction of wild hosts from in and around the field should be followed to keep the disease under check.
- With the initiation of the disease, spray the crop with fungicides like carbendazim (0.1%) or benomyl (0.1%) or mancozeb (0.25%) or chlorothalonil (0.2%) and repeat at 10 to 14 days interval.
3. Powdery mildew
- Powdery mildew is an important disease of this crop and under favourable weather condition causes significant yield reductions particularly if the infection takes place at in early stages of plant growth.
- Symptoms first appear as minute discoloured patches with thin fine meshwork of white mycelium arising at many places on the upper surface of lower leaves (Plate-3).
- These white patches soon join together to form larger white greyish powdery coating discernible on the severely affected leaves and in later stages the affected leaves turn yellow and finally drop.
Disease cycle and epidemiology:
- The disease is caused by Erysiphe cichoracearum DC.
- The conidia are single celled, hyaline, barrel-shaped and in long chains.
- The conidial dimensions vary with the physiologic race and the host.
- Cleistothecia are globose, dark with hyaline to dark brown and mycelioid appendages.
- They contain 8 to 18 asci and the asci are pedicellate, ovate to broadly ovate or ellipsoid.
- The number of ascospores per ascus is usually two, rarely three. The ascospores are one celled and hyaline oval to sub-cylindrical.
- The primary infection of the leaves is caused by wind blown ascospores (wherever cleistothecia are present) or by conidia formed on the earlier sown crop/ indoor cultivation in the neighbouring areas.
- In the process of infection, conidia germinate and after penetration, colonization of the entire leaf takes place.
- The conidia formed abundantly on primary infections are blown by wind and air currents and cause secondary infections and the cycle is repeated.
- The pathogen requires 60-80 per cent relative humidity for the development of the disease and dry conditions for the growth and sporulation of the fungus.
- With the initiation of the disease spray the crop with fungicides like wettable sulphur
- (0.25%) or dinocap (0.05%) or carbendazim (0.05%) or hexaconazole (0.05%) or difenoconazole (0.03%) and repeat at 10 to 14 days interval.
Other Important Diseases
- Root rot by Macrophomina and Rhizoctonia solani