Late blight is one of the most serious diseases of potatoes and tomatoes worldwide, resulting in significant yield and quality losses annually. In Alberta, late blight occurs infrequently, but can have devastating impacts in the years when it reaches epidemic levels.
“This disease is caused by a fungal pathogen called Phytophthora infestans,” says Robert Spencer, commercial horticulture specialist, Alberta Ag-Info Centre, Stettler. “The favourable conditions for disease development, combined with the presence of the pathogen, have resulted in multiple outbreaks of late blight in commercial, market garden and urban potato and tomato crops throughout parts of Alberta. For 2014, this disease continues to be a risk for these crops in Alberta.'
Late blight is an aggressive disease that, if left unchecked, can cause significant and rapid crop losses in fields, gardens, greenhouses and in controlled environment storages, e.g. potato bins. The risk of introduction comes from either infected transplant material (tomatoes or other host crops) or infected seed potato stock (either imported or carried over). During the season, if spore loads build up, there is a risk of introduction of the pathogen via wind-blown/storm carried transfer.
“Initial symptoms of late blight are typically noted on older leaves, appearing as dark, water-soaked areas (lesions), sometimes with yellow edges, that move in from leaf tips/margins, becoming brown and brittle within a couple days,” says Spencer. “Late blight lesions are not contained by the leaf veins. Lesions may also develop on plant stems and on potato tubers and tomato fruit. Late blight develops most quickly in warm, wet/humid conditions and can spread very rapidly through plantings. Plants may be rapidly defoliated, die and yields can be significantly reduced.”
Potato tubers may be infected by spores produced on the foliage that are subsequently washed into the soil. Infected tubers may have irregular, sunken lesions that are often first found around the eyes. The pathogen can penetrate into skin of the fruit or tubers, causing rot and discolouration of the internal tissues. The rot often has a reddish-brown colour. Late blight can spread from diseased to healthy fruit and tubers in stored tomatoes, in potato piles in storage and on seed potato pieces.
On the Prairies, late blight does not form an overwintering spore. Rather, the pathogen overwinters on living tissues, carrying forward from one season to another on infected seed potatoes, cull piles, volunteer potatoes or living host plants (e.g. tomato transplants).
In-season spread is by spores (sporangia) produced on infected tissues (infected transplants, volunteers, weeds and diseased crop debris). Spores spread within the fields by rain or water splash. Sporangia may also move short distances in soil water and spores may move between fields on equipment. Spores can move considerable distances on the wind.
When it comes to managing late blight, efforts centre on reducing the introduction of the disease into plantings, either by avoiding overwinter survival or by monitoring for infected plant materials that might be brought in from other areas.
“In early spring and into summer, the priority is for regular scouting and monitoring of emerging plants, and new plant material,” says Spencer. “Leaving potato cull piles or diseased materials in the open can lead to infection of healthy plants. Volunteer potato plants and solanaceous weeds should also be controlled.”
Late blight can be managed in commercial crops using protective fungicidal sprays (with rotating chemistries) applied at regular intervals when conditions favour disease development. The use of cultural practices, such as drip or furrow irrigation and the adjustment of plant stand density, can be effective in reducing the risk or rate of disease development in alternative crops or smaller stands.
“Infected plant material should be disposed of as soon as possible after detection, either by burying or freezing,” says Spencer. “If infected crop debris is composted, it should be covered with a tarp or soil until it has frozen to minimize the risk of spore survival and distribution. Killing potato tops can help to minimize tuber infection, as this encourages tuber skin set and stops top growth. Tubers can be harvested a couple of weeks after the tops are killed. Tubers should be heavily graded and culled before storage in an attempt to prevent entry of the disease into storage.”
Late blight is not a disease that occurs every year in Alberta. “There are other diseases that can resemble it, but are less serious in nature,” says Spencer. “By carefully managing any infected plant debris, people can help to minimize late blight outbreaks, and prevent it from overwintering and avoid a potential repeat infection of susceptible crops and weeds in future years. Late blight is a community disease, and will require the concerted efforts of everyone to get rid of it.”
Alberta Agriculture and Rural Development has prepared a number of print and electronic information resources to assist all levels of the horticulture industry as well as the public to recognize potentially infected material and understand the appropriate management steps to take. For more information on late blight, visit Alberta Agriculture's webpage or call 310-FARM (3276).