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

Fungal diseases

DAMPING OFF

• It is an important disease specially in tomato and also other vegetable crops like bell pepper, chillies, brinjal, cabbage, cauliflower, broccoli etc. for which nursery is raised for transplanting.
• The disease is fairly common in poorly managed nursery beds, incited often by the seed and soil borne pathogens.
• It is responsible not only for the poor seed germination and stand of seedlings but also for carry over of the pathogens to the field where transplanting is done.

Symptoms

The disease manifests in two phases
i) Pre-emergence damping-off and ii) post-emergence damping-off.

i) Pre-emergence damping-off:

• Failure of seedling emergence from the soil either due to seed rots or killing of young seedlings before their emergence from the soil.
• Resulting in patchy appearance of seedlings stands in the nursery in early stages (Plate-1a).

ii) Post emergence damping-off:

• This phase is characterized by toppling over of infected seedlings at any time after their emergence from the soil.
• The infected tissue initially appears to be water- soaked and soft.
• Subsequently, the stem at the infection points get constricted resulting in toppling over and mortality of the seedlings (Plate-1b).

Pathogen (s):

• Species of Pythium, Phytophthora, Fusarium and Rhizoctonia solani Kuhn are the most commonly associated pathogens.
• Among these fungi, the species of Pythium viz., P. aphanidermatum (Edson) Fitzp. and P. butleri Subram. are more common than others.
• The mycelium of Pythium is intracellular, consists of slender, coenocytic and much branched hyphae.
• Asexual reproduction is mostly by means of zoospores, that which are differentiated in a vesicle, kidney shaped and biflagellate, which later become encysted and germinate by forming germtube.
• In sexual reproduction, one male nucleus from antheridium passes into the oogonium through conjugation tube which then forms a thick wall spores known as oospore.

Disease cycle and epidemiology

• Most of the fungi responsible for this disease are both seed and soil borne (Fig.1.).
• The species of Pythium survives from one season to the other as mycelium in plant debris or oospores in the soil.

• Rhizoctonia solani survives in soil indefinitely in the form of sclerotia. Hyphae grow from these sclerotia and infect seedlings in the nursery beds. Species of Pythium attack the plants when they are in juvenile stage.
• Soil moisture near saturation stimulates mycelial growth as well as asexual reproduction.
• High soil moisture, pH 6.0 and heavy soils favour disease development.
• Pre-emergence damping-off is maximum at 20-25^o C while post emergence at 30-40^oC.
• The disease is further aggravated in ill-aerated soils with poor drainage having thick stand of the seedlings.

Management

• Change the nursery site every year.
• Either solarize the soil of the bed with transparent polyethylene (25 µm) sheet for 40-45 days during summer months or treat the soil with Formalin (5%) at least 20 days before sowing or apply bioagents like Trichoderma harzianum or T. viride (40 g/m2).
• Treat the seed with captan (0.3%).
• After seedling emergence from the soil, drench the bed with the mixture of mancozeb (0.25%) and carbendazim (0.1%) and repeat at 7-10 days interval.
• Give light but frequent irrigations.