Answer: Strawberry leaves can take on a reddish cast in the fall and winter because of cold night temperatures. If the leaves are just now showing red, it could be because of cold temperatures a few weeks ago. If it's been far too warm in your region for this to happen, you might be dealing with a fungal disease, instead. Red stele root rot is caused by the fungus Phytophthora fragariae. This pathogen overwinters in infected roots and can persist in the soil for many years. Red stele disease is worse in heavy soil or poorly drained planting sites. The fungus is most active in the spring and fall when soils are cold and wet.
Disease symptoms don't become evident until the bearing year (second season of growth). The fungus produces spores (zoospores) that are spread by water and mechanical means (machinery, tools, shoes, etc.). These spores infect and destroy the small hair roots giving a "rat tail" appearance to the remaining root system. The normally cream colored core (stele) of the roots turns a rusty red to dark-brown. The discoloration may include the entire stele or only the stele at the root tips. These symptoms are best seen in the spring by cutting a root lengthwise.
The fungus is restricted to the roots but any interference with root function is soon seen in leaf symptoms. New leaves are small and bluish green, while old leaves turn red. Plants are stunted, and little or no fruit is produced. Infected plants die out very quickly during dry weather.
To control the disease, select a well drained planting site with loamy soil. Purchase resistant plants from reputable growers. Resistance to red stele disease is a relative term, however. There are at least five distinct biological races of the causal fungus and all varieties of strawberries are not resistant to all races of the fungus.
Best wishes with your strawberry plants!
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