The most common disease of strawberries is gray mold, caused by the fungus Botrytis cinerea. You may have seen this mold in boxes of strawberries purchased at the supermarket. Under adverse conditions, such as cool, wet weather, the pathogen can cause the loss of much of your strawberry crop. Good cultural practices can help to limit these losses.
(Editor's Note: This article was originally published on June 27, 2008)
There it is, your strawberry bed, blooming away in the spring. There you are, anticipating a healthy crop of sweet, juicy red berries. Strawberry jam. Strawberry pie. Strawberry shortcake. But unseen in the soil where your berries are growing lurks the deadly fungus Botrytis cinerea. If you don't take steps to control it, Botrytis mold can ruin your crop.
The best way to ensure a mold-free crop of strawberries is to plant in sandy, well-drained soil in a sunny location. For those of us who don't have such soil, the best way is to plant in a raised bed or container. The Botrytis fungus develops most rapidly in cool, soggy soil, like the muddy clay in my garden, where drainage is a perpetual problem even though I have surrounded the strawberry beds with trenches to drain the water. This year, with a cool, rainy spring, has been ideal for the development of the disease, but so far, I have been relatively free of it. The berries that escaped and ended up in the trench, underwater, are hopeless, but the bed is still mold-free.
The Botrytis fungus overwinters in diseased plant material, such as infected leaves and rotted fruit. In the spring, in wet weather, the dormant spores (conidia) come to life. The strawberry blossoms are affected at this stage; many blossoms turn brown and die. The developing fruits may also be affected then, with the disease lying dormant until later in their development, but they can be infected at any stage by coming in contact with infected material, with other infected fruit, or wet ground, or by spores blown through the air. Bruises and injuries make the fruit particularly susceptible, but the fungus is also capable of infecting perfect berries. First, light brown areas develop on the fruit and spread. Immature fruit may turn brown and mummify, mature fruit may rot on the vine. In the final stage of infection as the fungus sporulates, a fuzzy gray mold covers the strawberry, and the spores escape to do more damage and keep the cycle alive.
Prevention is easier than cure with gray mold. Besides wet weather, a number of factors encourage the development of this disease on strawberries. Watering late in the day provides the damp overnight conditions that the fungus loves. When the plants are crowded closely together without room for air to circulate, the fungus flourishes. If fertilizer is applied early in the season, before the berries develop, the excessive growth of leaves may further cut down the air circulation around the fruit. Avoid crowded, overgrown rows and keep the strawberry bed thinned.
Chemical controls are most effective when applied during the blossoming period. Captan is usually recommended for home gardeners. Putting down a clean, dry mulch, like straw, can help by keeping the ripening berries off the damp, fungus-infected soil.
It is important to be able to identify the earliest stages of the disease. In the photo above, a light brown area is visible. This is a sure sign of infection. The brown shows up even more easily against the white color of the immature berry. A healthy strawberry has a shiny, glossy skin. Be suspicious of any dull-colored area, even if it is still red. If you cut open an infected berry, the flesh will be discolored, a light tan. Any suspicious berries should be picked and discarded immediately. Do not leave infected berries on the plant where the mold can spread to the rest of the bed. Be sure to keep picking your strawberries as long as they are producing. Overripe berries are particularly susceptible to mold. Letting fruit rot on the vine will only spread the disease.
If you have discovered the fungus in your strawberry bed, you should sort through each batch after you pick it, to make sure you have not overlooked any infected berries. Do not eat them. Or rather, do eat one, and you will immediately notice the foul taste. One of these in a batch of jam can ruin it. The gray mold can develop very rapidly in strawberries after they are picked, particularly if any of the berries are bruised. They should be processed as soon as possible. If you are not going to use them right away, be sure to refrigerate them quickly. Some people have been known to add a tsp of hydrogen peroxide to the water in which they wash their just-picked berries. I haven't done this myself, but I tend to think it couldn't hurt, especially if the berries are rinsed afterwards to wash off the bleach. If I am going to eat my berries fresh, I hull them and add a tablespoon or so of sugar to every quart before refrigerating. If I am going to use them for jam or cooking later, I freeze them after hulling.
Once your strawberry bed has been infested with gray mold, you will readily see the wisdom of renovating it. With Junebearers, this should be every year. Junebearers put out a lot of runners, which put out a lot of runners, and they spread vigorously beyond the bounds of their bed, if allowed. After they are finished fruiting, it is time to weed out the excess growth and clean out the bed to eliminate lingering fungus. Begin by mowing down the whole thing, leaving only the crowns of the plants intact. Then till or dig up the plants that have crept out to the edges of the bed, leaving only the center of each row. I run my tiller over the entire bed, knowing that enough plants will survive to establish a new one. Don't compost the material if there is reason to believe it carries the Botrytis fungus. Then rake out all the old straw mulch, if you used it, and the dead berries and leaves. Clean out everything that might be harboring the fungus.
At this point, although no one has advised it, I make an application of fungicide, on the general principle that it can't hurt, as well as fertilizer. Then dig out all the strawberry plants that survived the tiller, leaving only single rows of well-spaced plants. If your bed is subject to gray mold, you will want to make the rows particularly narrow, with wider spaces between them to allow for air circulation.
Most Junebearers are grown on the matted row system, because this is what the strawberries want to do. If you plant a single row with plants 18 inches apart, they will spread over the course of the next year to form a row about 18 inches wide. Planting these rows four feet apart will give you ample space between them after the runners have spread. In renovating a bed that has been afflicted with gray mold, you may want to leave nine inches or so between plants, instead of the six inches usually recommended. Crowding the plants will cause conditions that encourage disease.
Lately, some growers are advocating the use of black or red plastic mulch in growing strawberries. One reason for this is the suppression of gray mold, as the mulch protects the berries from contact. This practice is most suitable for the everbearer/day neutral berries grown on the hill system, in which all the runners are removed and only the mother plants remain to form large crowns. The plants are spaced closer together, about a foot apart, inserted through holes cut in the plastic. This method seems to be used most often where the strawberries are treated as annuals and discarded after harvesting the fruit. I have seen suggestions that Junebearers can also be grown this way, but I haven't tried it myself. Runners will not be able to take root through the plastic, so it would seem that they would have to be treated as everbearers.
If a bed of everbearers is infected with gray mold, it would probably be best to remove the plants entirely, clean the soil, and start over with new plants - at which time you might try the plastic mulch if the disease has been a problem. After several years, with any strawberry planting, it is considered advisable to start over with new, disease-free plants as the old strawberries lose their vigor. It is possible that you might want to consider re-locating the bed if mold has been a serious problem.
Finally, you might consider starting over with disease resistant plants. Unfortunately, however, there do not seem to be any varieties commercially available that are entirely immune to gray mold. 'Earliglow' is perhaps the most disease resistant of the Junebearers. Berries that remain firm at maturity and berries held high on their stems are other considerations that might help a new planting remain mold-free.