The hurry-up springtime of 1998 has brought strawberry harvest a couple of weeks earlier than normal. Gardeners and commercial growers have started picking berries by now in most of the state. The Plant Disease Clinic has begun to receive digital-camera images and samples of an all-too-familiar strawberry disease: leather rot.
Leather rot, caused by the soilborne fungus Phytophthora cactorum, causes immature and ripening fruit to discolor. The green fruit may develop hard, brown areas. Ripening fruit will develop a reddish or brownish purple discoloration and slight softening; eventually, the infected berry shrivels to a dry, leathery consistency, giving the disease its name.
The most distinctive and memorable aspects of leather rot are the taste and smell of infected berries. The smell has been compared to that of rancid motor oil, and the taste has euphemistically been termed "bitter." One infected berry can ruin and entire batch of jam; imagine motor-oil-flavored jam!
To control leather rot, you need to control water. The fungus spreads from soil onto berries by splashing raindrops and through puddles of standing water. The cheapest and most effective leather rot control is mulching. Oat straw is often used, but many other kinds of organic mulch will do the same job. The key is to provide enough mulch to create a barrier between the soil and the berries, so that rain can't splash one onto the other. The best time to mulch is early spring, just as the strawberry plants are resuming growth. Commercial strawberry growers and experienced backyard gardeners simply redistribute the straw mulch they've used on top of the planting all winter for freeze protection, raking it off the plants and into the rows between them. A cardinal rule for mulching strawberries: don't be stingy with the mulch. A barrier several inches thick is an excellent protector against leather rot, enabling most growers to escape the disease entirely.
If water stands in the planting for more than a few minutes after a rain, chances are the soil texture is very heavy (high in clay content). This type of soils usually makes for poor results with strawberries, which prefer well-drained soil. If you're stuck with clay, it's helpful to try growing on raised beds; even raising bed height by 4 to 6 inches will help the roots get enough oxygen to function, and keep most of the berries out of puddles after a rain. If you've got the time and energy - and money - you can also improve poorly drained sites by tiling them.
Several fungicides are labeled for leather rot control, but these are generally not available at local garden centers.
The take-home message for leather rot control is one word: mulch. If you do it thoroughly and consistently, you will likely have little if any leather rot to worry about.
This article originally appeared in the June 5, 1998 issue, p. 69.