Walking through an alfalfa field the other day, I saw numerous leaves infected with downy mildew.  Because only young leaves are susceptible, symptoms are near the top of the plant and easily visible. Early stage symptoms include a light green blotchy area on the leaf surface (Photo 1). In time the area will turn yellow and eventually may dry out (Photo 2). These diseased areas on leaves are usually localized.  In the morning when humidity is high, on the underside of the leaf you can see light gray or violet “fuzz” (Photo 3). Look at the “fuzz” with a hand lens.  It is the mycelium and spores of the fungal pathogen Peronospora trifoliorum, the cause of downy mildew in alfalfa. The main part of the fungus is growing in the leaf between the leaf cells but the part of the fungus that produces spores grows out through the stomata on the underside of leaves. Spores can then be dispersed by breezes or splashing rain to other alfalfa leaves.

Some infections become systemic which means that the growing tip becomes infected and the stem and all the developing leaves are infected. Internodes are shorter and the leaves are misshaped, often with their margins curling down. 

This fungus is an obligate parasite (meaning it can stay alive only as long as the plant tissue on which it is feeding is alive) so it usually doesn’t kill its host. However, individual leaves may die and leaves fall off the stem prematurely. When infection is heavy, the loss of leaves prior to harvest can reduce yield and quality.

Environmental conditions required for this disease are cool, moderate temperatures (from 40-80⁰F but with optimum around 65⁰F) with rain or high humidity because moisture is required for spore germination. With these requirements it is no wonder that in the San Joaquin Valley symptoms are usually observed in late winter, early spring and late fall but not much if at all in summer. 

Most varieties have some level of resistance and plants do not seem to have long term damage from infection. No studies have been conducted to determine if fungicide applications are justified.  If infection is widespread in early spring, harvest early if possible. Removal of infected leaves will reduce the number of spores available to infect new leaves. Once it turns warm, the disease is hard to find.

Just one more note: Current thinking by taxonomists is that this “fungus” Peronospora trifoliorum and related organisms, such as Phytophthora and Pythium spp., aren’t really fungi but more closely related to algae. But for practical purposes, I still call them fungi!