Although
major crop problems have not been commonplace in 2000, there have been a
lot of complaints both here and statewide about new seeding alfalfa fields
that look yellow and lack vigor. The
problem in at least some of these cases was confirmed to be Aphanomyces
root rot. The symptoms seemed
to be accentuated by the long stretches of wet weather earlier this
summer. At this point, let's review what we currently know about this
often forgotten alfalfa disease. Establishing
alfalfa on slowly drained soils has long been a challenge for Wisconsin
forage producers. The
development of varieties with high resistance to Phytophthora root rot and
the routine use of fungicide-treated seed have helped to overcome some of
the problems associated with wet soil alfalfa establishment.
In the early 1980's, a fungal disease called Aphanomyces root rot
was identified as limiting alfalfa establishment and yields on some wet
soils in Wisconsin. This was true even where varieties had high resistance
to Phytophthora root rot and fungicide-treated seed was used.
Until this time, the disease was only thought to have major
economic impact on peas. Several
years ago, the Fond du Lac Forage Council sponsored a project to quantify
the presence of Aphanomyces in area soils.
The study effectively confirmed that the disease is present in
almost all area soil types. Aphanomyces is a soil-borne fungus thought to be closely associated with Phytophthora to form a destructive alfalfa disease complex. Although Phytophthora root rot is often the dominant disease where soils are poorly drained, Aphanomyces can cause significant stand loss and reduce plant vigor, especially in the seeding year (Table 1). The use of fungicide-treated seed and varieties resistant only to Phytophthora root rot are not effective controls for Aphanomyces root rot. The extent of reduction in stand loss for both of these pathogens is largely dependent on environment. Heavy soils that remain saturated for extended periods offer the greatest opportunity for infection and significant disease development.
Symptoms The physical plant symptoms resulting from Aphanomyces infection can be difficult to distinguish from those of other alfalfa seedling pathogens. Infected seedlings express chlorotic cotyledons followed by chlorosis (yellowing) of leaf tissues. Roots and hypocotyls may become gray and water-soaked at first, then turn light to dark brown. Hypocotyls usually remain rigid and upright. This latter symptom is somewhat in contrast to Phytophthora and Pythium infections where hypocotyls typically collapse and the seedlings fall over. The foliage on infected plants is stunted, chlorotic, and appears nitrogen deficient. Established plants lack vigor and may be slow to regrow in the spring or following a harvest. Again, it's important to note that Aphanomyces root rot of alfalfa is frequently found in association with Phytophthora root rot. Acting together, the two related pathogens form a destructive disease complex. Although Phytophthora may be a problem in the absence of Aphanomyces, the reverse situation has rarely been documented. Environment, coupled with soil type and drainage, is the key factor governing infection by both disease pathogens. Race 2 strain discovered Within the past several years, the presence of a second strain or race of the disease has been confirmed. According to Dr. Craig Grau, UW-Extension Plant Pathologist, the alfalfa Aphanomyces population is composed of two virulence types; race 1 and race 2. Race 1 forms do not cause severe disease on commercial varieties with an R or HR rating. Race 2 isolates cause severe disease on race 1 resistant varieties in lab tests; and early field experiments indicate race 2 resistant breeding lines out-yield race 1 resistant varieties if race 2 is predominant. Most companies are working hard to develop alfalfa varieties with high levels of race 2 resistance. The issue of need for race 2 resistant varieties will surely come-up when such varieties hit the market and become a marketing strategy. Fields that continue to have annual or periodic establishment problems are candidates to investigate the value of race 2 resistance. Grau’s guess is that someday all varieties will have race 2 resistance and Aphanomyces will be under even greater genetic control.
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