Tip of the Week: Combating Fungal Diseases such as Late Blight

Posted in Gardening Tips on November 30 2009, by Sonia Uyterhoeven

Sonia Uyterhoeven is Gardener for Public Education.

Last week I blogged about the causes of late blight. This week we’ll look at ways to deal with it and other fungal diseases in general.

Here at the Garden, we thought we had missed the late blight since we made it through the initial onslaught unscathed. The problem with fungal spores is that some of them love to hang around. In late August we went out into the garden on a Monday after a heavy weekend rain and found a number of plants covered with white powdery spots.

On further inspection we found signs of brown lesions on stems, and several of the leaves were starting to darken into brown spots. Since this fungal problem progresses so rapidly, we decided that we wouldn’t wait to get the problem plants tested. I spent an afternoon filling garbage bags with diseased tomato plants, carefully cutting them up piece by piece so as not to unwittingly spread spores all over the place. Even for compost aficionados, it is advised to toss diseased plants into the trash rather than risk the disease surviving the composting process and infecting new areas of the garden.
 
After I disposed of the wreckage, I went back into the garden and doused the remaining tomato plants with a product called Plant Health Care Biopak Plus. It is a micronutrient treatment with beneficial bacteria that supports plant vigor (it is similar to spraying the plants with compost tea to protect them).
 
The following days were hot and sunny so we all gave a sigh of relief knowing that this fungal foe had a strong adversary in the pleasant weather.

What are the take-home lessons from all this drama? Whether disease, pest, or environmental, we have little control over some outside occurrences yet there is always a way to intervene and limit the damage.
 
In the case of late blight, the key is fast action so that the fungal spores are not allowed to develop. Good IPM (Integrated Pest Management) practices help: Inspect gardens on a regular basis to detect and assess any signs of pest or disease problems, catching them in the early stages before the situation explodes.

If you do see signs of disease, remove the infected foliage and fruit immediately and dispose of it in the trash. Cut back infected plants midday, once the foliage has a chance to dry. The bright ultraviolet light will help to destroy any spores that are stirred up. In any disease situation, never work with wet foliage—you will only intensify the problem as water is an ideal conduit for spores. At the end of the season, remove plant debris from infected locations.
 
It is generally a good practice to rotate your crops on a three- to four-year rotation, but it is especially imperative after a bout with late blight. Don’t plant any member of the Solanaceae family in your tomato bed the following year—not eggplants, peppers, or potatoes.
 
A few good gardening practices during the growing season will help stave off the threat of fungal diseases: Water your garden early in the day so that the foliage is dry by the evening and direct the water to the base of the plant, avoiding the foliage.
 
Caveat: While it is important to take decisive action in cases such as late blight, it is equally important not to overreact when you see other signs of plant damage. For instance, there are other diseases that affect tomatoes, but they are relatively innocuous. Also, plants experience a general wear and tear as they go through the season and end up with blemishes—our version of bumps and bruises—which are strictly cosmetic.