Late blight of Solanaceae vegetables
- Economically important disease caused Irish famine in 1845
- Disease of cool moist area but has adopted to subtropical climate.
- In Nepal about 15% yield loss has been reported due to late blight.
Etiology: Phytophthora infestans
- Class- Oomycetes, Order- Peronosporales Family- Phythiaceae
- Mycelium hyaline, coenocytic hyphae, intercellular with single or double club shaped haustoria
- Facultative saprophyte, Compound interest disease, Pandemic disease
- Symptoms generally appears on the above ground parts of potato plant but later on the underground parts such as tubers are also affected.
- Small brown patches appear on the leaf tip or margin and spread downward and inward in favorable weather condition.
- Moist weather kills the entire leaf within 1-4 days and give offensive smell.
- In severe condition symptom extend to the stalk quickly and the entire crown may fall.
- In dry weather the spots remain small, brown and dry while in warm muggy weather, the color of spots rapidly changes to black and the stem are being infected.
- Early attack of this disease results death of top which will reduce size and number of tubers formed.
- Primary invasion of the tubers by sporangia or zoospore falling from the infected leaves and reaching the tubers through soil causing rotting, a dry rot or wet rot.
- Skin of tubers become slightly sunken and dark in color. Rusty brown appearance can be observed just beneath the skin of tuber followed by brownish dry rot.
- In storage wet rot phase of tuber is Common.
- Slimy soft decay is generally the effect of secondary bacteria.
Pre- disposing factors
- Excessive humidity ˃90% coupled with suitable temperature 10-25ºc for germination of sporangia by germ tube or by zoospore
- No sporangia produce below 90% RH.
- Maximum spread of disease occurs if sporangia produce zoospores (12-13ºc) or directly by germ tube at 24ºc.
- For epidemic condition low temp, in a wet atmosphere for formation of sporangia and their germination by zoospore and slight rise in temp for the growth of germ tube and subsequent development of mycelium in the host.
Forecasting of disease
- Night temp below dew point for at least 4 hours
- Minimum temperature 10ºc
- Cloud on the next day
- Rainfall during the next 24 hours at least 0.1mm
Increased rate of N fertilizers have two fold effects on late blight
- As dense foliage provide favorable moist microclimate though atmospheric weather is dry
- Young plants and young leaves of older plants are susceptible
- Smaller tubers are infected more because of lenticels being resistant in large sized tuber.
Infected tuber in cool storage serves as the primary source of infection. The mycelium survives in dormant condition with the potato tubers when such tubers are sown, the infection start from the seedling stage. Dispersal of sporangia and zoospore may take place through any part of epidermis of leaves and young stem and initiate secondary infection. Tubers in the field are infected if spores from the blighted tops are washed by rain water. Healthy tuber gets infected when come in contact with diseased plant parts at the time of harvest.
- Selection of healthy tubers.
- Burn crop debris.
- Early sowing to escape the wet, cool and humid environment.
- Wider spacing than recommended.
- Application of 100:90:60 kg NPK/ha.
- 15 cm high riding at the time of earthing reduces tuber infection.
- Delay harvesting as it reduces viable inoculum in dried foliage and infection to tuber.
- Sorting potato from blighted fields.
- Resistant variety – Janakdev, Khumal Rato-2, Khumal Seto-1, NPI 106, Hybrid-14.
- Tuber treatment for 90 min in1:1000 mercuric chloride solution saves tuber from secondary rot in storage.
- For prophylaxis or preventive 2.5gm/lit Mancozeb (maneb+zineb) in all parts of plant 7- 10 days interval for 4- 6 times.
- For curative Metalaxyl 1.5gm/lit 10-12 days interval for 3-4 times.
Source : Crop disease and their management