Crops:Potato and Tomato
Why is it a problem? Late blight is the most important disease of potato in Bhutan. The pathogen is capable of destroying an entire unprotected potato crop within one to two weeks under the right weather conditions. Losses can be largely avoided through intensive management, although fungicides are often required. Late blight has also severely affected recent attempts at commercial-scale tomato production, but it is not yet known how easy it will be to manage.
Where and when is it a problem? Late blight appears every year in all major potato-growing areas. It is most severe above 2000 m asl where wet conditions occur in the first 80 day of potato growth. It has also been reported as a pest of tomatoes at higher altitudes when grown later in the season (Zhemgang) and in greenhouses (Haa).
- Leaf blights can expand to kill leaves within 1-4 days, can spread to petioles then spread and may cause entire plants to collapse.
- Water-soaked spots appear on leaves which increase in size and turn brown and black. Lesions can have chlorotic (loss of colour) borders, but rapidly expand and become necrotic (death of cells).
- In humid conditions infections result in a visible growth (white mould) at the leading edge of lesions, especially on the lower surface of leaves and even on stems.
- On stems, late blight causes brown lesions that look greasy. The lesions commonly occur at the junction of the leaf and stem where water may accumulate or on clusters of leaves at the top of the stem. Stems break at infected points and the plant topples over. .
- Symptoms on tomato foliage are similar.
- On tomato fruit, late blight produces dark brown, firm lesions which may enlarge and destroy the entire fruit. Symptoms appear on both green and ripe fruits. As in potatoes, lesions can often be followed by soft rot and disintegration.
- Infected tubers show irregular, small to large, slightly depressed lesions. Lesions are copper brown, reddish or purplish skin and can spread to the entire surface and into the tissue. The boundary between diseased and healthy tissue is not well-defined, unlike in early blight. Lesions can be examined further by cutting the tuber and a reddish-brown dry rotted tissue that extends < 1 inch into the tuber can be observed.
- Infected tubers are often invaded by soft rot bacteria which rapidly convert adjoining potatoes into a smelly, rotten mass that must be discarded. This can make observations of late blight symptoms difficult.
Confusion with other pests: Late blight can be confused with early blight. Early blight lesions appear dark brown with concentric rings within a definite margin. Lesions are often surrounded by a yellow halo. Late blight lesions do not produce concentric rings with definite margins. Herbicide damage can also be confused with late blight, but closer examination will show a lack of lesions and other symptoms of fungal infection.
Life history: The pathogen mainly survives in the soil but can also survive in infected tubers (potato) and tomato seeds. During the season under favourable conditions, the pathogen in the soil or in plant debris reproduce rapidly, producing spores (sporangia) which are splashed from the soil to the plants or from plant to plant. The spores can also be carried by water in the field or swim towards the root zone to infect tubers. Infections generally begin in tuber cracks, eyes or lenticels. Once infected by late blight pathogen, tubers are susceptible to infection by other pathogens. Inside the infected plant, the pathogen produces another type of spore which has a thick wall. These thick-walled spores can survive in the soil for a very long time.
Dispersal: The pathogen is dispersed by wind-borne spores which can be released early in the new growing season from infected tubers left in the field from the previous season. Spores can also spread through the soil by water to infect tubers. Infections can also be spread through movement of infected seed potato and tomato seeds.
When can damage be expected? Severity can vary a lot between years, even in the most susceptible areas. Epidemics are favoured by wet and humid springs. They typically occur during heavy monsoons when the weather remains cloudy and misty with continuous drizzle for several days. Such conditions can be exacerbated in greenhouses, through the use of sprinklers that maintain humid conditions.
Hosts: Potatoes and tomatoes.
Management of late blight at mid to high altitudes requires an integrated approach. This includes careful cultivar selection, strict on-farm hygiene practices, good pre-planting preparation, constant surveillance and intensive management during the first 80 days from planting.
- Monitoring to guide management decisions is critical as disease severity and timing can vary greatly from year to year on any one farm. Act (spray) immediately when first symptoms are noticed.
- Inspect the field frequently (every two or three days) from first spring rains until 80 days after planting (or when potatoes are fully formed). Examine the leaves (especially to observe white mycelium on the underside of the leaf) and stems throughout the canopy to look for early symptoms (including the black/brown lesions). Give special attention to moister parts of the field.
- Late blight infections can sometimes occur slightly earlier on certain farms within a growing area. This can serve as an early warning, reminding other farmers to be extra vigilant.
Effect of variety
Potatoes vary considerably in their resistance to late blight. If possible, the most resistant varieties should be favoured in areas susceptible to late blight. Desiree is the most susceptible, Khangma Kaap has some resistance, and Kufri jyoti and Yusi Kaap are moderately resistant. The newly released NKK (Nsephyel Kewa Kaap) is thought to be the most resistant. However, late blight can evolve rapidly so resistance status can change quickly.
- Use only healthy, disease-free seed potatoes and tomato seed from reliable sources.
- Where possible set planting times so potatoes are maturing and less affected by disease once the monsoon starts (typically late June). At lower altitudes it could be December to February. This is not possible at higher altitudes where it is too cold to plant before February-March.
- Plant tubers at recommended spacings as crowding favours disease multiplication. For table potatoes, rows should be 70 cm apart and plants within a row 20 cm apart. For seed potatoes, the recommendation is 60 cm and 15 cm.
- Cut any infected haulms to minimise the spread of late blight to other plants and to tubers. Collect and bury (at a depth of 1 metre) or burn infected tubers and vines.
- Remove volunteer plants immediately after emergence and destroy.
- Hilling helps in early weed control and minimizes tuber infections from sporangia.
- Avoid crop rotation between potato and tomatoes as they are both late blight hosts.
- Apply fertilizer at recommended rates. Excessive nitrogen fertilisation increases canopy cover, delays maturity and may reduce yield. It also induces susceptibility to late blight.
- Avoid sprinkle irrigation as it creates humid conditions that favours late blight development. Use of drip irrigation in greenhouses is recommended.
Seed potato harvest and storage
- Destroy all infected foliage a few days before harvest, remove from field and destroy (burn) to reduce spore load for the next year.
- Sort and remove diseased tubers before storage. Don’t source seed potatoes from infected plants. Infected seed potatoes are the main source of infection in potatoes.
- Wash field equipment and machinery after use.
- Avoid movement of animals in the field.
- Fungicide is often needed to protect crops during humid periods in the first 80 days from planting. In areas where outbreaks are frequent two to three sprays may be needed in most years.
- The first spray is largely preventative and should be applied as soon as the first symptoms are seen. This is typically during the first or second week of May at 2,000-2,500 m asl and late May or the first week of June at > 2,500 m asl. However, it could be earlier if Spring is unseasonably wet, or delayed if conditions are dry.
- Subsequent sprays should be a minimum of 10-14 days apart and be based on field monitoring. Spray only if symptoms are detected in the field. They may not be necessary every year.
- Spraying is not necessary once tubers are fully developed.
- Spray both sides of the leaves. The first, preventative, spray should be Mancozeb (2 grams/litre of water), the second spray Metalaxyl (2 grams/litre of water) and any subsequent sprays Mancozeb. Metalaxyl should only be used once in a season as it can easily induce resistance.
- Sometimes localised infections (hotspot) within a field can be a guide to start spraying fungicide. In such a case, remove the infected plants in and around the hotspot area and then spray the whole field as above. However, the effectiveness of such a practice depends on early detection of symptoms.
Image acknowledgements: NPPC.