Take-all was severe in continuous wheat rotations throughout the study. Take-all was just as severe following millet as after soybean. Results from the 1996 and 1997 showed that a one year rotation with canola prior to wheat had a significant effect on suppression of take-all root rot. Wheat grain yield was the same, and in most rotations test weight, and 1,000 kernel weight were the same as the non-diseased controls following canola. This was a greater yield improvement than anticipated because of the severe damage from take-all in continuous wheat during all three years. Yield components were significantly lower in rotations with continuous wheat. The possibility that the rapid decline in take-all after canola may be due to compounds released during decay of canola tissues is being investigated. Assays in a controlled environment chamber for take-all on wheat seedlings grown in soil from field plots were similar to results from field trials. The incidence of infected plants and root rot severity were greatly reduced on seedlings grown in soil from rotations with canola. Data for disease incidence and severity from the field prior to wheat harvest were similar to data from seedling assays. The beneficial value of canola in wheat rotations for take-all control has been incorporated into Extension recommendations in Georgia.
The population of Hessian fly on wheat was below economic thresholds during 1995. During 1996 and 1997, winter infestations were significantly lower when canola was rotated with wheat. By 1997 Hessian fly infestations were greater following millet than after soybean. Spring infestations were not affected by the previous crop. False chinch bugs were present in canola in the first season but could not be assessed in 1996 due to the severe winter freeze which killed canola. Millet stands were reduced by false chinch bugs following canola in 1996 and 1997. True chinch bugs, which often damage millet seedlings after small grains, were not found. Thrips populations were higher on soybean seedlings after canola than after wheat, but thrips are not damaging to soybean. Fall armyworm, southern green stink bug, and leaffooted bug attacked whorls and seed of millet but their populations were not influenced by crop rotation. Insect pest populations on soybeans were not affected by the preceding winter crop. Only soybean loopers and velvetbean caterpillars were above economic thresholds. More lepodopterans were found on soybeans following rye. The primary effect of rotation sequence was on seedling insect pests.
The stand of soybeans was reduced 20% or more following canola each year which could not associated with insect damage. Pearl millet stands were also significantly lower after canola but not wheat. This is the first evidence that canola may have an allelopathic effect from breakdown products in canola stubble on these crops in a doublecrop rotation.
Rotation effects were significant for pearl millet panicle counts, leaf blight, and stalk and neck rot during the 1996 season. There was a trend for leaf blight and stalk and neck rot to be less severe in pearl millet following canola than following wheat. Seedling stand was positively correlated with leaf blight and stalk rot. Rotation effects on pearl millet diseases may have been obscured by variation in stand density within subplots. Minor effects on pearl millet stand establishment, retention of green foliage, stalk and neck rot, smut, and yield were observed in these rotations. Seedling stands 3 weeks after planting were lower following canola than wheat in 1995 and 1997. Foliar chlorosis and necrosis were slightly less severe in pearl millet following canola than wheat in 1995 and 1996. Stalk and neck rot was also less severe following canola in 1996. Grain yield was highest following canola in 1997 but may be an artifact of excessive stand density. A trend toward increased smut severity in plots continuously planted to pearl millet was observed in 1997.
Canola diseases were not significant during 1995 and could not be assessed because the crop was killed by low winter temperatures in 1996. After two seasons, there were no differences in severity of stem canker on soybeans due to crop rotation or the inclusion of canola or pearl millet in the rotation. Sclerotia stem rot and black leg did not become serious during the three-year period, probably as a result of loss of the crop in 1996. The rotations will be continued in 1998 to assess rotation effects on these diseases.
Soybean yield was reduced following canola probably because of allelopathic effects from the decaying canola stems and roots. Various foliar diseases were present but did not cause economic loss in any year. Stem canker increased in severity early in the 1997 season in continuous soybean rotations but did not affect seed yields. Additional data will be collected in 1998 on rotation effects on stem canker. Canola and pearl millet were compatible with soybean in the rotation. No changes in disease management is foreseen to incorporate these crops into rotations with soybean.
Analyses of the income returns from the rotations was done by developing cost and return budgets for each crop by year using plot yields and field operation and input costs realized by the Southwest Georgia Branch Experiment Station. The analysis shows that crop failure and crop prices influence rotation effects on returns. These influences can magnify negative relationships such as reduced yields of soybeans behind canola, and/or shrink positive benefits such as take-all reduction on wheat from having canola in the rotation. High soybean prices and wheat prices may make the reduction in soybean returns following canola less than the increase in wheat returns from having canola in the rotation ahead of wheat. In contrast, beneficial but costly winter cover crops with no income (rye) and/or low income potential neutral crops such as millet can make a rotation unprofitable or add no positive economic benefits. However, the low income of millet could be an improvement over soybeans after canola. The results indicate the current vulnerability of the new crops canola and pearl millet. Improvements in varieties adapted to the region and improved yield potential will make these crops more profitable.
Video footage showing the results of the rotations on diseases and insect pests to date in field plots have been made. Information on the procedures used and the biology and damage caused by the various pests have been documented. Aerial footage showing the plots during May to document the effect of rotation on take-all of wheat has been recorded. Film will be edited to complete the educational video based on the study. Several training programs and presentations at various meetings have been completed and additional presentations will be made in 1998. Audiences varied from small acreage to large-scale growers, extension specialists, and researchers throughout the Southeast. Several technical and nontechnical publications have been completed. Refereed journal publications are being prepared from the research on rotation effects on take-all, insect populations, and pearl millet diseases and crop yields. A comprehensive publication on the project will be prepared as a College of Agriculture Research Bulletin.
Disease and insect pests became serious problems in the Southeast when wheat and soybean were grown in continuous double cropping and minimum tillage systems. Therefore, the objectives of the project are to:
1. Enhance double cropping systems with minimum tillage in the southeastern U.S. by expanding crop rotations which can be profitable and which can reduce diseases and insects.
2. Incorporate improved cultivars of the emerging crops canola and grain pearl millet into minimum tillage systems.
3. Determine the optimal rotation system to manage diseases and insects in canola, pearl millet, soybeans, and wheat.
4. Demonstrate the usefulness of these rotations to growers on a commercial farm and at a major regional farm exposition site.