Controlling Late Blight in Potato and Tomato Gardens
Late Blight is the single most damaging disease of potato. In Saskatchewan, the typically warm and dry summers limit disease development and late blight is not a common problem. However, the cool and rainy conditions encountered across the province in 2009 and 2010 were ideal for late blight, reminding us that wet conditions can lead to substantial loss from blight. Reduce late blight by planting disease-free stock and diligently monitoring for symptoms. Late blight typically affects only potatoes and tomatoes in Saskatchewan, but peppers, eggplant and petunias are also susceptible.
On potato and tomato leaves, late blight symptoms first appear as water-soaked lesions surrounded by a pale green halo (Photo 1). The lesions run across the veins, as opposed to the disease "early blight" where the lesions are typically confined by the leaf veins. Infected stems and petioles will turn dark brown or black (Photo 2). In dry weather, late blight lesions stop growing and turn dark brown and brittle. Early in the morning or on wet days, a white cottony growth may form on the underside of infected leaves (Photo 3). Stems of infected plants weaken and droop, giving a characteristic flagging appearance to the canopy. A strong decay odour often accompanies widespread infection.
Infected potato tubers develop a reddish brown discolouration just beneath the skin (Photo 4). Lesions can also develop on tomato fruit, usually as a bronze or coppery coloured lesion at the shoulder of the fruit (Photo 5). Infected potatoes and tomatoes rot quickly, either on the plant or once in storage.
Late Blight Disease Development
Sources of infection include infected seed potatoes, infected tomato seedlings and spores blowing in from adjacent diseased gardens, fields or cull piles. If infected potato tubers are planted, late blight will likely be present on the emerging plants. Infected plants produce spores that easily spread to adjacent plants. Potato tubers are infected by spores that wash from blight-infected leaves into the soil. Spores can also be transported in the wind for several miles, or be spread by water runoff from infected areas and by moving machinery. Plants can die within days of being infected when conditions favour rapid late blight development (15 to 20 C and 90 per cent relative humidity).
Late Blight Management
It is very difficult to control late blight once symptoms are evident. Prevention is key. The following practices focus on limiting sources of infection and modifying the environment to reduce disease development and spread:
Disease-free seed is the first line of defense against late blight. Avoid planting seed potatoes that may potentially be carrying late blight, as this is the primary way that disease spreads from season to season. Purchase seed potatoes from reputable suppliers. Inspect the seed prior to planting and discard any tubers that appear to be infected. Culled (discarded) potatoes should be bagged, buried, composted, or otherwise disposed of; if they are allowed to sprout they will be a source of disease. The disease is not spread in tomato seed but transplants may become infected early-only plant disease-free tomato seedlings.
While no potato cultivars are truly resistant to late blight, russet cultivars tend to be more resistant than red types. A number of tomato cultivars have improved tolerance to late blight: Blight resistant varieties are listed in seed catalogues.
Late blight infection is favoured by excessive vine growth caused by over-application of nitrogen fertilizer. Water plants early in the day so foliage has time to dry before nightfall. If possible, water the soil around the plants and not the foliage. Tomatoes should be pruned and staked to promote airflow through the canopy. Hilling potatoes reduces the chance of spores washing from infected leaves down into the soil. Potato vines should be completely dead and removed from the garden prior to harvest, otherwise spores can spread from vines to tubers during harvesting. If possible, leave tubers in the ground for two weeks after vines have been removed to allow for blighted tubers to rot so they can be easily identified and culled out at harvest. Tubers near the soil surface are most likely to be infected; therefore, it is prudent to discard tubers that show signs of greening. If more than five per cent of the harvested tubers have late blight, it is unlikely that the crop can be successfully stored.
Whether a home gardener or a commercial grower, diligent monitoring for late blight symptoms is crucial. Identifying an outbreak early allows time to prevent disease spread and protect the remaining crop. Frequently (every two to three days) examine the leaves and stems throughout the canopy to look for early symptoms (refer to SYMPTOMS).
Commercial producers have more fungicide options available to them. Home gardeners have access to copper-based fungicides from garden stores. These fungicides work preventatively and should be applied before symptoms are evident. To be effective, fungicides must be applied throughout the growing season, after rain events and/or as new foliage develops, and full coverage of the canopy is necessary. Always follow the manufacturer's instructions when applying fungicides. An integrated approach to managing late blight, including the practices described above, is advised as it is more effective and reduces dependency on fungicides.
Removal of Diseased Plants
Suspect plants should be immediately pulled, bagged, removed from the garden and destroyed. Plants in the vicinity of the infected plant are also likely infected and ideally should be removed or at least closely monitored for signs of disease.
Potato tubers and tomato fruit going into storage should be carefully inspected. Any suspect produce should be destroyed. Monitor the stored produce carefully, especially for the first few weeks after harvest, as late blight has the potential to spread within storage and destroy the entire lot.
Vigilance is key to managing late blight-Keep a close eye on your crop and listen for reports of late blight occurring in your area. For more information, visit the University of Saskatchewan's Gardenline website or call 306-966-5865.
- Dr. Doug Waterer, Department of Plant Sciences, University of Saskatchewan 306-966-5860
- Glen Sweetman, Provincial Specialist- Greenhouse and Nursery Crops, Saskatchewan Ministry of Agriculture 306-787-6606.