With the cooler and wetter late season weather conditions that we have experienced in Pennsylvania, we have started to receive some reports of different foliar diseases affecting forage grasses. Based on the current reports, our focus in this article will be on how to identify the different rusts that may be seen on forage grasses, as well as what symptoms and signs you should look for to identify Stagonospora leaf blotch. Additionally, please consult the following two additional references to help in the identification of different forage grass diseases: “Diseases in Orchardgrass” and “Diseases of Forage Grasses in Humid Temperate Zones”. In general, the primary recommendation in cases of severe infection by a forage grass disease at this stage of the growing season would be to consider early cutting of the forage, maintain proper and optimum soil fertility, as well as discuss with your seed rep varieties or cultivars well-adapted for your location looking towards the future.
There are several different types of rust that may affect forage grasses. To differentiate these rusts, we can use a combination of pustule type, location and color. For example, for crown rust, look for scattered, bright orange-yellow pustules that develop on upper and lower leaf surfaces (Figure 1). The pustules (uredinia) are round to oval and may contain masses of orange-yellow spores that are exposed when the leaf epidermis ruptures. Uredia may also be found on leaf sheaths. When infection is severe, leaves turn pale yellow and wither.
With stem rust, the epidermis is ruptured by uredial pustules that will have orange-red spore masses (urediospores) (Figure 2). These spore masses can be dispersed to other plants. In mature plants, brown-black, oblong to elongate telia form and this is termed the black rust stage of the disease.
Stripe rust will have uredia that are lemon-yellow in color and which can be seed on both sides of the leaf. Typically, long stripes will form between the veins of the leaf. As the season progresses, the pustules will turn black in color as telia are produced.
Finally, with leaf rust, round to oval, yellow-brown to red-brown pustules can form on the upper leaf surface (Figure 3). Telia of the leaf rust pathogen are rarely found later in the growing season.
Another foliar disease that may be impacting the forage grasses is Stagonospora leaf blotch, also known as Stagonospora leaf spot (Figures 4 and 5). Often, you will also see this referenced in terms of Septoria and a proper differentiation of the two diseases requires a microscopic examination of the spores. In terms of symptoms of Stagonospora leaf blotch, these typically begin as small, blackish-brown to deep purple spots that can be elongate in shape. In severe cases when there are many lesions, these coalesce, which causes the leaf to brown and die. Browning often can be seen developing at the tip of the leaf or along the margin and form a long streak. A sign of the pathogen is to look for small fruiting bodies (pycnidia) in dead areas of the leaf, which is how the pathogen overwinters. Cool, wet weather favors infection.
Figure 1. Crown rust symptoms (Photo source: Penn State Extension).
Figure 2. Stem rust of forage grasses (Photo source: Penn State Extension).
Figure 3. Leaf rust of orchardgrass (Photo source: A. Tajimi, Japan).
Figure 4. Stagonospora leaf blotch (Photo source: A. Collins, Penn State University).
Figure 5. Close-up photo of pycnidia forming on the leaf of orchardgrass (Photo source: A. Collins, Penn State University).