Using genetic crosses between single chs mutants of Ustilago maydis inoculated into maize (Zea mays) seedlings, two classes of double mutants affected in genes coding for chitin synthetases were isolated: chs3/chs4, and chs4/chs5. Analysis of the mutants showed almost no change in their phenotype compared with wild-type strains. Growth rate, effect of stress conditions, dimorphic transition and mating were not affected. The only salient differences were increased sensitivity to osmotics at acid pH, and decrease in chitin synthetase activity, especially when measured with CO2+, and in chitin content. Most significant was a decrease in virulence, although this appeared to be due a factor unrelated to CHS genes. These data can be taken as further evidence that multigenic control of chitin synthetase in fungi operates as a safety mechanism to guarantee fungal viability in changing and hostile environmental conditions.

Living mulch is a cover crop which is intercropped with a cash crop to protect against soil erosion and structural deterioration without taking land out of production. To avoid interference which reduces cash crop yield, the mulch requires management techniques which minimize resource utilization during the critical period of crop development without killing the mulch outright. The experiments reported here examine non-chemical management innovations in a sweet corn-white clover intercrop system which allow for clover regrowth and also contribute to the N requirement of the corn. In 1986 clover was suppressed by mowing or partial rototilling. Yields of corn in plots where clover was rototilled were comparable to yields of clean cultivated corn and superior to yields from plots where clover was mowed or unsuppressed. In 1987 rototilling was compared at 3 timings after corn emergence. Highest corn yields were obtained by rototilling the well-established clover at 2 rather than 4 or 6 weeks after emergence. Yields from rototilled plots in 1987 exceeded those from clean cultivated plots or plots where clover was unsuppressed. After rototilling, the clover reestablished via stoloniferous growth emanating from a narrow strip of roots which passed between the tiller tines. In both years corn leaf N concentrations were highest in the rototilled plots. In 1987 clean cultivated corn appeared N deficient and had a greater incidence of corn smut than corn from the rototilled clover plots. Multiple corn row arrangements which facilitate access to the mulch were compared at a constant plant population to conventional single rows on 76 cm centers. There were no differences in yield between single and double rows (152 cm centers) in either year, while the triple-row arrangement (228 cm centers) reduced yield in 1987 due to the low productivity of the plants in the middle row of the 3 rows.