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Symptoms and signs
Symptoms
Symptoms of Pythium blight are evident during warm, humid weather when turfgrass leaves are wet for at least 12 hours. The disease is particularly severe when daytime temperatures exceed 28°C (82°F) and night temperatures fail to fall below 20°C (68°F). Initial symptoms appear as dark green to purple water-soaked leaves that aggregate into circular or irregularly shaped patches in turfgrass swards (Figures 1-9). These patches can range in size from < 1 to > 20 cm (0.4 to > 8 in.) in diameter. No distinct lesions are formed on infected leaves, which often feel slimy or greasy when rubbed between two fingers. Patches of infected grass can enlarge and coalesce, causing severe damage to lawns, golf courses and athletic field turf (Figures 10-11). Pythium blight can also appear as streaks in areas where the fungal-like pathogen is spread by flowing water or mowing equipment (Figure 12). In addition to foliar blight, crown, stolon and root rot are frequently associated with Pythium blight.
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Signs
Pythium blight is also called "cottony blight" which refers to the cotton candy-like growth of white, fluffy aerial mycelium growing from infected turfgrass foliage (Figures 13-15). The mycelium is most abundant when infected leaves are wet, e.g. in the morning after a night of rain or heavy dew. Aerial mycelium produced by Pythium spp. is similar in appearance to mycelium produced by Sclerotinia homoeocarpa, which causes dollar spot, and Rhizoctonia solani, the causal agent of brown patch. However, the latter pathogens produce foliar symptoms that are distinct from those produced by Pythium spp. Even though the aerial mycelium of the three pathogens is similar in appearance, microscopic observation of the hyphae reveals major differences. The most notable difference is the absence of septa in hyphae of Pythium spp. (Fig. 16A), whereas Rhizoctonia and Sclerotinia spp. are septate. In addition Rhizoctonia produces hyphae that branch at right and acute angles to the main hypha (Figure 16B). The branch hyphae appear slightly constricted at their origin, and a septum is present near the branch origin. Right angled branching is also a characteristic that distinguishes R. solani from S. homoeocarpa (Figure 16B and C). The diameter of S. homoeocarpa hyphae is generally larger than R. solani hyphae.
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Copyright © 2004
by The American Phytopathological Society