SEED TRANSMISSION OF PYRENOPHORA-TRITICI-REPENTIS, CAUSAL FUNGUS OF TAN SPOT OF WHEAT

Seed transmission of Pyrenophora tritici-repentis, a common foliar pat hogen of wheat, was investigated in soft white winter wheat cv. Franke nmuth and found to be non-systemic; the emerging coleoptile was infect ed externally by hyphal growth from the infected pericarp. Hyphae from the infected coleoptile then infected the first and second seedling l eaves as they emerged. Coleoptile symptoms ranged from tiny brown stre aks or spots to large, brown necrotic areas accompanied by cracking an d distortion of the coleoptile. Small brown spots sometimes occurred o n the first and rarely on the second seedling leaves, often accompanie d by leaf distortion. Pseudothecial initials of the fungus were presen t within or on the seed remnants. Seed transmission efficiency was as high as 92% in vitro and 60% in potting soil outdoors. Seed infection did not affect germination in vitro, but slightly reduced emergence in potting soil. Seedling weight and height were reduced significantly. In potting soil, seed infection also resulted in delayed plant growth and increased tan spot severity at later stages of plant development. Under controlled conditions, seed transmission efficiency and incidenc e of pseudothecia on seed were negatively correlated with seed germina tion temperature in the range of 9 to 21 degrees C, whereas frequency of recovery of the fungus from symptomatic coleoptiles and leaves was positively correlated with seed germination temperature. These results suggest that infected seed may serve as a source of inoculum for tan spot epidemics and for dispersal of strains of the fungus to new areas .