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Vascular occlusion associated with abiotic stress is a causal agent of silvertop in perennial grasses

Abstract: Silvertop (sterile heads) of perennial cool-season grasses is common in aging stands grown for seed production. It is associated with yield reduction, but the causal agent(s) has not been determined. A study was conducted to examine the cytology of grass tillers with and without silvertop. A series of cross sections of stems from field samples of Kentucky bluegrass and meadow bromegrass (where signs of insect injury or fungal infection were absent) revealed abnormalities in the tissues above the last node in tillers exhibiting silvertop, while cells in the stems of healthy tillers appeared normal. Most cells from stems exhibiting silvertop became irregular and distorted, and eventually lost their integrity and shape altogether. At the terminal nodes of these stems, the sieve plates of xylem vessels were occluded with unidentified material, preventing the passage of water to the seed head. The xylem and phloem cells in the leaf sheath and blade surrounding the silvertop stems appeared normal. Spherical bodies in x-section, believed to be fungal mycelium and spores, were occasionally found above the point of tissue necrosis above the last node. Very few spherical bodies were observed below the symptomatic tissue, and none were found in the terminal node. The blockages observed in the water-conducting vessels to the seed head appear to have been induced by the plant as a response to abiotic stress, rather than by external biotic factors such as insects or pathogens.