June 17, 2007
Xianming Chen and Richard Koenig
Recently, physiological leaf spot (PLS) has been showing up in some winter and spring wheat fields. I have heard that people mistreated PLS as stripe rust and tried to use fungicides to reduce PLS symptoms. Because PLS and stripe rust are controlled by different methods, it is important to distinguish PLS from stripe rust. Similarly, it is important to tell PLS from other spot/blotch diseases caused by fungal pathogens.
Physiological leaf spot is a complicated disorder and is sometimes used to describe problems of which the causes are not known. However, a typical PLS is a physiological and genetic disorder that expresses spot symptoms on leaves in fields with a deficiency of chloride. Physiological leaf spot symptoms start with tiny chlorotic (mosaic) spots, which can grow up to spots of a quarter inch in diameter. The early symptoms resemble those of the early infections of rust and some other pathogens. Early spots sometimes have a halo pattern, but mature spots usually have a distinct margin. PLS spots are usually round to oval in shape, but in severe situations, spots connect to each other, appearing to be irregular. Strategies to manage PLS include growing tolerant varieties, rotation with other crops or fallow and the application of chloride. When the crop is growing, application of chloride is the only way to reduce PLS symptoms. Fungicides, such as Tilt, Quilt, Quadris, Headline, and Stratego, which control stripe rust and other fungal diseases, do not control PLS.
As shown in Figure 1, it is easy to differentiate PLS symptoms from the typical symptoms of stripe rust.(Figure 2). It is also easy to differentiate PLS from resistant reactions of wheat to stripe rust. After the stem elongation stage, stripe rust produces white necrotic stripes without or with limited rust pustules as shown in Figure 3. Wheat crops showing resistant reactions to stripe rust usually do not need to be spayed with fungicide while those showing susceptible reactions should be sprayed with a registered fungicide such as Tilt, Quilt, Quadris, Headline, or Stratego when stripe rust develops to 5-10% severity. Unlike PLS, stripe rust cannot be reduced by the application of chloride.
It is more difficult to distinguish PLS from spot/blotch diseases caused by fungal pathogens. For comparison, pictures showing symptoms of fungal spot/blotch diseases are shown in Figure 4. The major difference between PLS and these fungal diseases is that PLS does not spread from field to field while the latter group of diseases spread because they produce infectious spores. These fungal spot/blotch diseases are not as common as PLS and stripe rust in the Pacific Northwest. Fungicides that control rusts usually control the fungal spot/blotch diseases.
Another fungal disease commonly found in wheat and barley fields is powdery mildew (Figure 5). The fungus produces fluffy, white to grayish mold. The disease is most common in fields under irrigation. Powdery mildew usually does not cause significant yield losses in the inland Pacific Northwest and, therefore, fungicide application to control powdery mildew is generally not necessary. However, the appearance of powdery mildew may be sometimes used to justify foliar sprays of fungicides when the variety is susceptible to stripe rust and stripe rust is potentially a problem. Fungicides that control stripe rust also control powdery mildew.
Figure 1. Symptoms of physiological leaf spot (PLS).
Figure 2. Susceptible reactions of wheat to stripe rust, showing yellow to orange-colored rust pustules containing and releasing powdery rust spores.
Figure 3. Resistant reactions of wheat to stripe rust, showing necrotic stripes without or with limited rust pustules.
Figure 4. Symptoms of Septoria blotch (left), tan spot (center), and spot blotch (right)produced by different fungal pathogens.
Figure 5. Powdery mildew of wheat showing fluffy,
White to grayish mold growing on leaf surfaces.