Tomato Late Blight

We’ve often spoken to gardeners who just shake their heads despairingly when we mention growing tomatoes. The Tomato Late Blight fungus really has hit Coastal gardeners hard and, from what we hear, has now even made its way into the Okanagan. Most new varieties are resistant to the common fungal diseases, but late blight (Phytopthera infestans) is dreadful here on the Coast. The leaves develop brown spots which eventually cause the whole leaf to turn brown and drop off. If conditions are right for the fungus, the entire plant becomes diseased and dies quickly.

Although late blight has been around for many years, infections have become much more severe in the 1990s. Scientists at Simon Fraser University recently identified a separate “A2” type which previously was only found in Mexico. This A2 type is much more aggressive and is capable of surviving for long periods on old vines and even in the soil.

How your plants become infected

The fungus spreads itself by forming spores that are carried by the wind. When the spores land on either potato or tomato plants, it needs the right conditions in order to germinate. The clue to understanding how to deal with it is to know that the fungus only grows on tomato plants that are wet for over 48 hours at a time. The dampness can come from rain, heavy dew, condensation in a greenhouse or tent or even your sprinklers. Additionally, there is some feeling (although this is still being debated) that plants initially become infected in the spring by splash-up caused by raindrops falling on bare, infected soil.

Contrary to popular thought, late blight does not travel inside the plant to infect other areas. Spores must fall on the fruit in order for them to become infected. However, once one part of the plant is infected, the spores that form there are much more likely to infect other parts of the plant.

What you can do to minimize loss from late blight

Once you know how your plants become infected you can plan proper control measures.

Keep plants dry by placing them under an overhanging eve, on a balcony or porch, or under a structure that you create. The protection you give them must allow for excellent ventilation, and you must be able to get in to prune and pick the tomatoes. If possible, do not water overhead. Try to keep the leaves as dry as possible by only wetting the ground around the plant. Keep tomato plants regularly watered and fed because dramatic fluctuations in temperature, soil moisture and soil fertility may cause fruit disfigurement in tomatoes.
This fungus overwinters in old tomato and potato debris and some weeds. Practice 4 year rotation, tomatoes following the cole crops in an area that was limed and manured last year. Do not plant in a bed that had potatoes last year! Gather all diseased plants, leaves and fruit and potatoes and put them in the garbage, not the compost.
Minimize initial infection by using a mulch around your plants and trimming the bottom leaves. Mulches such as grass clippings or straw absorb the impact of raindrops and reduce the chances of soil being sprayed onto your plants.
Another preventive measure is a fixed copper spray, often sold as Bordeaux Mix. (Now called “Bordo Spray” Spray the whole plant according to label directions. The caution with this spray is that copper can accumulate in the soil and slow plant growth after several years, so it is important to plant the tomatoes in a different part of the garden each year, and to change the soil in planter boxes as well. Leave at least 1 day between the time you spray and the time you harvest. Copper compounds are generally considered a regulated input by organic certification bodies, meaning they should be used as interim solutions only, and alternatives (keeping the plants dry) should be sought.
Since the disease strikes in the rains of late summer, choosing varieties that will ripen before the deluge is also a control measure. See the listings below and plan your harvest! Perhaps the best and surest advice we can offer discouraged tomato growers is to concentrate on the determinate varieties which can be easily grown in attractive pots and planters up under the house eaves. There is less trouble with staking and covering; a wire cage support, regular watering of the rich soil and full sunshine are all that are needed. Determinates ripen much earlier than the indeterminate varieties, so they are ready for summer salads simultaneously with other salad ingredients.

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