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

STALK ROT

This is an important disease of cauliflower seed crop (late varieties). Once this disease gets introduced in a field, it is very difficult to raise a healthy crop in the same field without taking appropriate control measures.

Symptoms:

• The symptoms of the disease start appearing with the earthing up of plants.
• The infection starts from the lower most leaf petiole touching the ground.
• The infected leaves lose their turgor during day time and droop down to the ground, but regain turgidity during night or early morning.
• The yellowing starts from tips of the older leaves downwards which shed, prematurely.
• In most of the cases, mid-rib and petioles at a point touching the soil, show small, discrete to large irregular dark brown to black necrotic lesions (Plate-3a).
• The lesions are covered with fluffy growth of extrametrical fungus under cool and humid weather.
• Rotting from petioles advances to the stalk where dark brown to black spots are produced which girdling whole of the stem at ground level.
• The stalk rot progresses towards the curd and occasionally whole of the pith portion up to the forks of curd branching get completely rotten (Plate-3b).
• The pith and the curd also rot, giving way to large cavities lined inside with fluffy mycelium and numerous sclerotia of the causal fungus.
• Under cool humid conditions, the mycelium emerges out and can be seen sticking to affected portion of the plant.
• With the progress of the disease, curds are also affected and show brown to dark brown rotting which may start from any portion of the curd, but generally from the center.
• The affected tissue becomes soft and mushy bearing numerous sclerotia.
• Inflorescence is also affected during the months of April-May in hilly areas.
• If there are plenty of rains during bolting and seed setting, the fungus progresses fast and engulfs whole of the branches and inflorescence where mycelium can be seen hanging out with sclerotia sticking to it.
• However, if the weather is dry, the mycelial growth is restricted only up to branches. affected branches become dry and bear shriveled seeds.

Pathogen:

• The disease is caused by Sclerotinia sclerotiorum (Lib.) de Bary.
• Mycelium in culture as well as on host surface is white, cottony, branched, consisting of closely septate hyphae.
• Micro-conidia are produced on short lateral branches of the vegetative mycelium in chains.
• When the food supply is exhausted and the vegetative growth ceases, the hyphae with granular protoplasm collect in small dense masses and form sclerotia.
• These sclerotia are first white in colour, then turn black and are round to irregular in shape.

Disease cycle and epidemiology:

• The fungus overwinters in the form of sclerotia in soil as well as in the diseased plant debris (Fig.2).
• In the presence of proper humidity and light conditions, sclerotia germinate by forming apothecia, which in turn form asci and ascospores.
• If the spores, upon escaping from the ascus, lodge on a susceptible host, a new infection may originate.
• Mycelium from sclerotia is also capable of infecting cauliflower.
• During cool and moist weather fluffy mycelium appears on the host surface and when food is exhausted, the mycelium coagulates and starts forming sclerotia.
• The fungus can infect the host at a temperature ranging from 0 to 25 ^oC with an optimum at 15 to 20 ^oC and high humidity (95-100%) also favours the disease development.
• Application of nitrogen aggravates the disease.

Management:

• Follow long crop rotations with paddy and maize.
• Remove infected leaves at weekly intervals.
• Soil amendment with oil cakes like sunflower and mustard and mulching with pine needles and sunflower inflorescence also reduces the disease incidence.
• Some antagonistic fungi like Trichoderma harzianum, T. viride, Gliocladium virens and Coniothyrium minitans have been found promising in managing this disease.
• These fungi either inhibit the development of new sclerotia or destroy the developed ones by colonizing them.
• In chemical control, sprays of fungicides like carbendazim (0.1%) or thiophenate methyl (0.1%) or mancozeb (0.25%) or combination of mancozeb (0.25%) and carbendazim (0.05%) effective in reducing the disease.
• Initiate sprays immediately after earthing up and repeat at fortnightly intervals.