from several sources, including mineralized soil organic matter, symbiotically fixed N, and N incorporated into Research on the effects of N and B applications on soybean [Glyplant tissue. Demand for seed N is highest from the R5 cine max (L.) Merr.] yield is limited. Experiments were conducted kg ha Ϫ1 applied from seedling emergence to R1 in-Holland Rd., Suffolk, VA 23437; and M.M. Alley, Dep. of Crop and creased yields by 720 kg ha Ϫ1 , averaged across all treat-

Simulation is used to compare water use, yields, and the relative profitably of fixed and towable center‐pivot sprinkler irrigation and subsurface microirrigation for a corn (Zea mays L.) and soybean (Glycine max L.) rotation. Mean water use is somewhat lower on corn for microirrigation compared to sprinkler irrigation while water use differs little on soybean. Yield responses to irrigation differ little between the two systems. Relative profitability of systems depends on size. At 75 acres, subsurface microirrigation is the most profitable system while the towable center‐pivot system is most profitable at 150 and 300 acres. The towable center pivot is more profitable than the fixed center pivot for all system sizes. Microirrigation is more profitable for smaller systems because of its lower per‐acre investment cost and lower pumping cost. Towable center‐pivot irrigation is more profitable for larger systems because its peracre investment cost declines with increasing size.

Without irrigation, crop yields are usually water‐limited on the acid soils of the southeastern USA. Economically feasible irrigation systems that require less water are needed. As part of a feasibility study of subsurface microirrigation for corn (Zea mays L.) production, the effects of irrigation line spacing and irrigation water amount were investigated in a 5‐yr study at Suffolk, VA. The soil was a Uchee loamy sand (loamy, siliceous, thermic Arenic Hapludult) with inclusions of Emporia loamy sand (fine‐loamy, siliceous, thermic Typic Hapludult). Treatments were three line spacings (0.91, 1.82, and 2.74 in apart, buried 0.38 m below and parallel to the crop row), low and high water amounts (about 41 and 62% of average pan evaporation, respectively), and a nonirrigated control. Nonirrigated grain yields ranged from 3.12 Mg ha−1 (1987) to 9.73 Mg ha−1 (1989). Irrigated grain yields ranged from 8.27 to 12.02 Mg ha−1. Except for 1987 (the driest year), there was no significant difference between the low and high water application amounts. In 1987, corn irrigated with the higher rate yielded 34% more. Grain yields were highest with irrigation lines placed under each row (narrow spacing) and ranged from a low of 8.55 Mg ha−1 (wide spacing) in 1989 to a high of 12.25 Mg ha−1 (narrow spacing) in 1990. Increased amounts of irrigation water were needed with the wider line spacing to obtain corn grain yields comparable with the other two spacings. It is unlikely that the lowered yield or increased irrigation water required with wide line spacing could offset additional installation costs for the narrow spacing. Irrigation lines spaced in alternate corn row middles (1.83 m) or under the third row (2.74 m) are recommended.

No abstract

The development of Sclerotinia blight, caused by Sclerotinia minor Jagger under various environmental conditions, was studied in field plots of peanuts (Arachis hypogaea L.). The peanut plant canopy was modified to produce desired environmental parameters. The m&cations included the thinning of canopy foliage to allow air circulation that would decrease canopy humidity and the addition of water-filled troughs under an unthinned canopy &at would increase humidity. Canopy relative humidity and soil moisture under the canopy was decreased by canopy thinning. Following infection by S, minor, the number of infection foci and disease development was reduced in the thinned canopy; however, thinning also reduced pod yield. Disease development was not increased, nor was yield affected by the addition of the water-filled troughs which increased humidity levels in the canopy. Soil moisture and canopy light interception were important variables in multiple linear regression models for the disease severity index and longest lesion length in the thinned and unthinned-trough plots.