Q What is potato or tomato blight? What causes it?A When gardeners on the West Coast refer to “blight”, they usually mean the disease called “late blight”, caused by Phytophthora infestans. This disease is famous as the cause of the Irish potato famine in the 1800’s. Phytophthora infestans is not a fungus or a bacterium or a virus. It belongs to a group of organisms called “protists”, although they are still commonly referred to as “fungi”. They are also called “water moulds” because they thrive and produce spores under humid, moist environment and cause infection only when free water is present on the plants.Late blight occurs in the Interior too, but “early blight” is more common than late blight in this drier region. Early blight is caused by a fungus called Alternaria solani. Both diseases cause leaf and stem lesions and fruit rot, so are sometimes confused. Although they are called “early” and “late” blight, both diseases can occur at any time from spring to fall if weather conditions are favourable. Early blight generally develops at warmer temperatures than late blight, which generally prefers cool, wet weather. However, with the appearance of new strains of the late blight pathogen, some strains can be expected to cause late blight at warmer temperatures.Late blight also occurs on other solanaceous plants such as eggplant, nightshade and occasionally on peppers.
Q Does the late blight fungus infect stems as well as leaves?
A Yes. On tomato, the first symptom on plants is often a brown/black lesion on the stem or petiole. Leaves develop large brown/black blotches, often starting at leaf margins. In humid weather and in early mornings, a fuzzy mould can often be seen on the underside of the brown/black blotches or on the stem lesions. This fuzzy growth contains structures called sporangia containing spores of the pathogen. On tomato fruit, infection causes a brown/black, leathery rot. It may become soft and mushy if invaded by secondary organisms.On potato, large, soft, greyish-green to tan areas first appear on leaves. Shoot tips may turn brown or black. Brown to black lesions also develop on stems and leaf petioles. In humid, wet conditions, a grayish white, fuzzy mould can be seen beneath the spots on the underside of the leaves. In wet weather, the entire plant may die back to the ground. If the disease is present at harvest, and rain occurs, tubers may be infected resulting in a firm brown rot that starts from the skin. Eventually, infected tubers will rot from late blight and secondary soft rot bacteria.
Q How does late blight spread?
A In cool, wet or humid weather the pathogen produces structures called “sporangia”. These sporangia can travel up to 20 kilometers in wind or wind-blown rain. Rain-spread sporangia can cause infection even in a garden where tomatoes or potatoes have not been grown before.Sporangia can also move in ground water, runoff or in watering splash from plant to plant in the garden. If they are contained in a water droplet which does not dry up for a few hours, they will release tiny spores called “zoospores” which swim through the water, attach themselves to the leaf or stem tissue and cause infection.Sporangia and zoospores don’t survive long. However, the pathogen can survive mild winters on small bits of un-rotted or un-frozen plant debris in the soil. The pathogen can form thick-walled resting spores called “oospores” when two different mating types happened to be present on the same plant. Oospores can survive for many years in soil without living on plant debris.
Q Is the late blight disease worse than it used to be?
A Yes. Every year, temperature and rainfall affect the timing and severity of late blight disease. But, in recent years, new and highly aggressive strains of late blight have appeared across North America, including B.C. One strain, in particular, affects both tomatoes and potatoes; another strain affects only potatoes. Home garden potato and tomato are often left untended and diseased plants are allowed to remain in the garden through the summer and fall. This leads to high spore levels and more potential for recombination of the two mating types to produce new strains. Home gardeners are asked to destroy severely infected tomatoes and potatoes to prevent the development and spread of new strains. Severely diseased plants should be burned (where permitted), buried or sent to a landfill. Oospores may carry over in soil and may survive the composting process.
Q Why does the blight seem to be worse (or milder) in my garden than in my neighbour’s? Or worse in one part of the garden than another?
A Rain-spread spores can cause spotty outbreaks. Also, disease development depends on the temperature and humidity around your plants. Plants in warmer, drier, sunny spots will have less disease. For example, plants that receive the morning sun will dry off more quickly from nightly dew and fog. Plants grown in a high moisture-holding soil or planting mix w