Disease and Insect Management Using New Crop Rotations for Sustainable Production of Row Crops in the Southeastern United States

Take-all. Take-all was severe during the first year with wheat yield reduced >75% below that of the healthy controls. The percentage of infected wheat plants in 1996 was reduced significantly in rotations where canola was planted in 1994. The average root disease severity score was about one-half as much as with canola planted the previous year as continuous wheat. Disease severity was less in the continuous wheat rotation in 1996 with pearl millet as the summer crop in place of soybean, but he difference was not statistically significant. Incidence of disease (percent infected plants) was not affected with nearly 100% of plants infected for both rotations (Table 1).

The incidence of infected rye plants was one-half that of wheat and disease severity was much reduced in 1996 (Table 1). After two years of rye (rotation 12), the incidence and severity of take-all in wheat in 1997 did not differ from continuous wheat (Table 2). This agrees with previous results for the susceptibility of rye. However, wheat grain yield was significantly higher following rye grown as a winter cover crop than continuous wheat. Previous research (Rothrock and Cunfer, 1991) showed that wheat following a rye grain crop had as much damage as continuous wheat. Additional data from other rotations were not available in this study. Therefore, these results need further validation. Take-all incidence and severity on wheat and rye plants increased each year in the noninfested control area, but were similar to rotations with canola the previous year. Rye, whether grown as a grain or a cover crop in fields with severe take-all, is less reliable to reduce take-all than other non-host crops.

Take-all caused severe yield reduction in the continuous wheat rotation sequences (rotations 1 and 9) in 1996 and 1997 (Tables 1 and 2). Grain yield in rotations in which canola was grown as the winter crop in 1995 did not differ from the controls, ie wheat harvested from portions of the test site not infested with Ggt. Similar results were found for grain test weight and 1,000 kernel weight (Table 2). These results show that canola can be planted as a cash crop for one season in sites heavily-infested with the take-all fungus and reduce inoculum levels below an economic threshold. Currently oats is the only fall-planted small grain that reduces take-all damage in a following wheat crop. Oats is a much less profitable crop than canola is expected to be. Therefore, canola can diversify crop choices for a grower with a potential increase in profitability and serve to control take-all in wheat.

Soybean maintains Ggt inoculum at a high level. Comparisons are made in this study to compare pearl millet to soybean as the summer crop for their influence on survival of Ggt and yield components in continuous wheat production. In 1995 no differences in yield components were found when wheat followed pearl millet or soybeans. In 1996 grain yield was not higher following millet but test weight and 1,000 kernel weight were significantly higher compared with soybeans as the summer crop (Table 2). However, in 1997 there were no differences in wheat yield or disease parameters between the two continuous wheat treatments with either continuous soybean (rotation 1) or continuous millet (rotation 9) as the summer crop (Table 2). Millet in the rotation does not reduce take-all.

Data from the field on take-all were corroborated with assays for development of take-all on wheat seedlings grown in soil collected from field plots on five sampling dates (Table 3). Data from the fourth sample date are lower than expected, possibly because seedlings were removed from growth chambers before symptoms were at the maximum. Results from April, 1997 (date 5) are more comparable to earlier results. Where no wheat or rye has been grown during the first two years (rotations 2, 4, and 6), take-all severity declined to less than 0.7 compared with 3.2 with continuous wheat (0-4 scale with 4 being most severe) in 1996. Disease severity for wheat in 1996 following canola in 1995 (rotations 3, 8, and 11) declined to 1.9 or less. Disease severity also declined in the seedling assay to 1.1 or less in rotations with wheat in 1995 and canola in 1996 (rotations 5 and 7). Incidence and severity has stabilized in continuous wheat (rotation 1 and 9) with incidence remaining >90%. Similar to field results, millet or soybean did not differ in their effect on Ggt survival and take-all severity. The incidence of take-all is in general agreement with the disease severity results. In the rotations with no wheat, incidence of seedling infection decreased to 31-60% by the third sampling date compared with 100% in continuous wheat. In rotations with wheat followed by canola, incidence was <80%. When wheat followed canola, incidence at the third sampling was 85% or less. By 1997, rotations with canola reduced take-all incidence to less than half that for continuous wheat in most cases (Table 3). Insects - Winter Crops Wheat. Because the study began in fall 1994, no rotational effects were established in the 1994-95 crop season. Hessian flies were not recovered from wheat in the February samples but averaged (+SD) 14.2 + 7.7% infested tillers and 0.39 + 0.07 immature per tiller in the spring samples. In 1995-96, winter infestations were significantly greater where wheat was the previous winter crop compared with wheat where canola was the previous winter crop (Table 4). Spring infestations were not significantly affected by previous crop. In 1996-97, both winter and spring infestations generally were greater in wheat following wheat than following canola (Table 5). This contrast was significant only for the spring infestations. Previous summer crop did not affect Hessian fly infestations in 1995-96, but infestations were greater following millet and than following soybean in 1996-97. Canola. Canola grain yield averaged 1619 + 49 kg/ha in 1995 and 1516 + 98 kg/ha in 1997 and was not significantly different between plots in either year (Table 6). Because a severe freeze in December 1995 killed canola in all plots, no grain was produced in the 1995-96 season. Plots were replanted in March 1996 and plants were bolting in mid-May when they were killed to prepare for planting summer crops. False chinch bugs were present in canola plots at harvest in 1995, but population numbers were not determined. No insect results were collected in 1995-96. In 1997, one generation of tarnished plant bug, Lygus lineolaris, developed on plants between flowering and harvest (Table 7). Large number of false chinch bugs also were present immediately before harvest (Table 7). Insects - Summer Crops Grain millet. Millet plant stands were not significantly different between cover crops in all years, although plant numbers were lower following canola than wheat or rye in 1995 (Table 8). False chinch bug were present in most millet plots following canola in 1995 with infestations in 2 of 8 plots exceeded 10 bugs/ft2 (Table 8). Millet seedlings in these plots were stunted and killed by bug feeding resulting in extensive gaps in the stand. Chinch bug numbers were much lower on millet in 1996 and 1997 than in 1995. Millet was attacked by the fall armyworm during the whorl stage, but infestations were not affected by previous winter crop in any year (Table 9). The two seed feeding insects were the southern green stink bug, Nezara virdula (L.), and leaffooted bug, Leptoglossus phyllopus (L.). Both insects sucked seed contents causing seed to be shriveled and deformed. The southern green stink bug numbers generally were not affected by previous crop in any year except on the second sample data in 1995 when stink bugs were more prevalent in millet following canola than wheat or rye (Table 9). Leaffooted bugs also were not different between winter crop treatments in any year. In 1997, large numbers of the corn earworm, Helicoverpa zea, were feeding on millet seed heads on September 4, but a viral epizootic virtually eliminated earworms by the September 17 sample. Large numbers of southern green stink bugs also caused considerable injury to seed on millet seed heads in 1997. Soybean. Soybean plant stands were significantly lower following canola than wheat or rye in all years (Table 10). False chinch bugs were somewhat more abundant following canola than wheat in all years, but little visible injury to seedlings was observed in any year. Thrips also were more abundant on soybean following canola than wheat in 1995 (Table 10). Predominant defoliating insects in soybean were the green cloverworm, Plathypena scabra (F.), soybean looper, Pseudoplusia includens (Walker), and velvetbean caterpillar, Anticarsia gemmatalis Hubner. Other phytophagous insects included the bean leaf beetle, Ceratoma trifurcata (Forster), spotted cucumber beetles, Diabrotica undecimpunctata howardi Barber, threecornered alfalfa hopper, Spissistilus festinus (Say), and stink bugs (>95% were N. virdula, others were brown stink bugs, Euschistus spp.). Predominant predatory arthropods were big-eyed bugs, Geocoris sp., damsel bugs, Nabis spp., and various spiders. Populations of soybean loopers and velvetbean caterpillars were above economic threshold and produced defoliation levels exceeding 50% in all plots by late September in 1995 and 1997.

Winter crops did not significantly affect populations of any phytophagous taxa any sample date in 1995 (data not presented). However, when averaged across the entire season in 1995, more soybean loopers and consequently total lepidopterans occurred on soybean following rye than canola (Table11). Numbers on soybean following wheat were not different from numbers on soybean following canola or rye. Likewise, soybean predator populations also were not significantly affected by winter crop on any sample date, except damsel bugs were more abundant following canola and wheat than rye on the first date that damsel bugs were collected in 1995. In 1996, winter crop did not significantly affect the seasonal average number of any arthropod taxa except that southern green stink bugs were were less abundant following rye and canola than following wheat (Table 12). However, stink bug number were very low in all plots in 1996. In 1997, winter cover crop also did not significantly affect seasonal average number of any arthropod taxa (Table 13). Over all years, winter crop did not consistently affect any foliage-inhabiting arthropod taxa on soybean.

Pearl millet diseases - 1995. Rotation had a significant effect on stand counts (P<0.0001). Pearl millet following canola tended to have lower stand counts than that following wheat or rye (Table 14). Rotation had a small effect on panicle counts (P=0.09), which were highest following rye. Yield was affected by rotation (P=0.05) and was lowest in rotation 4, which also had the lowest stand counts. Pearl millet yield following canola did not consistently yield less than when following wheat, as evidenced by rotation 8. Rotation had a small effect (P=0.10) on leaf blight. Pearl millet following canola tended to have less chlorosis and necrosis than that following wheat. 1996. Rotation effects were significant for panicle counts, leaf blight, and stalk and neck rot (Table 15), although there were few consistent effects due to the preceding crop. There was a trend for leaf blight (symptoms were consistent with those caused primarily by Exserohilum rostrata) and stalk and neck rot (caused primarily by Fusarium roseum and Bipolaris setariae, respectively) to be less severe in pearl millet following canola (rotations 2 and 7) than following wheat (rotations 3, 8, 9, and 11). The effects may be obscured by the high stand density of rotation 6, which resulted in relatively higher values for leaf blight and stalk and neck rot. Correlation coefficients among yield components and disease data indicated that stand density had a significant effect on yield components and disease within subsample areas (Table 16). Seedling stand was positively correlated with leaf blight and stalk rot. The higher stands within subplots of rotation 6 probably resulted in greater severity of leaf blight and stalk rot compared to disease in rotations 2 and 7 (Table 15). The 1996 rotations indicate that the previous crop had no consistent effect on pearl millet grain yield, however, high seedling density promoted leaf blight and stalk rot. 1997. A calibration error in planting the pearl millet plots resulted in excessively high stand density. Problems associated with the high stand density included extreme variation in plant development within the rows. Panicles ranged in size from about 8 cm to over 30 cm long, and many plants failed to form panicles. With the intense competition for resources, plants matured rapidly after pollination and before differences in leaf blight and stalk and neck rot could be evaluated. In spite of the excessive stand, seedling stand was lowest in millet following canola (Table 17). The effects of canola may be somewhat long-lived, since even plots planted two years after canola (rotations 4 and 5) tended to have lower stands than those in which canola had not been planted at all (rotations 9 and 12). The high stands in rotations 9 and 12 resulted in greater panicle numbers. Seedling stand and panicle number within subsample areas were significantly correlated (R=0.51, P<0.0001). Smut was significantly greater in plots that had been planted continuously to pearl millet over the three years (rotation 9). Smut severities were lowest in plots that had not been previously planted to pearl millet (rotation 5), but the differences with the other treatments were not significant. Grain yield was greatest in plots following canola (rotation 3). Higher yields following canola have not been observed in previous years, therefore it is likely that the greater yield was the result of a slight advantage in growing conditions resulting from the lower stand density. Canola diseases. No symptoms of Sclerotinia stem rot were observed on canola during 1994-1995. This was probably a result of dry weather conditions during the flowering period. A period of several days of saturated soil is required for germination of sclerotia and production of ascospores. To complete the infection cycle, this period must be followed by a period where the foliage and flower petals remain wet. This apparently did not happen during the critical flowering period in 1995. The crop was frozen in the seedling stage in December 1995 and could not be replanted until March 1996. Thus, there was no canola present during the usual infection period and by the time the replanted crop was flowering it was too hot for infection. As a result, no infected plants were found during 1996. Very light and scattered infections of white leaf spot, downy mildew, and Alternaria pod infections were observed each year but were not severe enough to be rated. The anticipated disease problems of canola have not developed at this stage of the study. This may be due, in part, to the loss of the second year canola crop. In 1996-97 the disease situation was similar to previous years. Light and very scattered development of white leaf spot and downy mildew was observed on leaves and stems, and Alternaria was present on a low percentage of pods prior to maturity. The major diseases, blackleg and Sclerotinia stem rot, were not observed at any stage of crop development. Both have been observed at the Plains Station previously and stem rot was observed at other locations on the station in 1997. The absence of these diseases from these plots in 1997 was unexpected and is difficult to explain. Two factors probably contributed to this situation, a relatively thin stand and the loss of the 1995-96 crop to freezing. The canola stand was adequate for good yield but was thinner each year than the stand usually obtained in commercial fields. The thin stand density and relatively wide plot borders may have allowed sufficient air circulation within plots to promote rapid drying of plant surfaces and thereby reduced the potential for pathogen invasions. The 1995-96 crop was not reestablished until mid-March. This three month period is the normal invasion time for both blackleg and stem rot. The absence of the crop during this period may have been as effective as a one year rotation in interrupting the development of these diseases. Soybean diseases. Stem canker developed to moderately severe levels in both 1995 and 1996 (Table 18). The low yield of Ga 81-2057 is probably primarily a result of damage from stem canker. In the absence of disease Ga 81-2057 and Brim should have produced similar seed yields. Both Brim and DP 105 are considered to be susceptible to stem canker, whereas Ga 81-2057 is extremely susceptible. The stem canker ratings of Brim and DP 105 indicate that disease severity was higher in 1996 than in 1995. There was no significant difference in stem canker severity in either year that could be attributed to previous crop sequence. The low yield of soybeans following canola in 1996 cannot be attributed to differences in severity of stem canker. Although the soybean stand following canola was significantly reduced in 1996, this was also true in 1995 when the yields were not different. In 1996 a comparison was also possible between soybeans following two successive winter crops of canola and those following only one. That comparison indicated no significant difference in stem canker severity or in yield. One possible explanation involves the replanted canola that had to be mowed at the early flowering to permit planting of the summer crops in 1996. This immature canola could have been much higher in glucosinolates than normal mature canola and could have released much higher levels of phytotoxic decomposition products. There is no evidence that inclusion of pearl millet as a summer crop and canola as a winter crop has increased the severity of any disease of soybean. Conversely, there is also no evidence that inclusion of these crops into the double-cropping system has reduced any of the soybean diseases. Changes in disease intensity may become evident after additional cycles of the rotation are completed. In 1997, brown spot, downy mildew, and anthracnose were present throughout the season but were very light and scattered. A few plants had symptoms of red crown rot and a few were killed early in the season by an unidentified cause. None of the diseases reached a severity that would be expected to influence seed yield. Stem canker incidence and severity were higher than previous years. The incidence of visible symptoms just before maturity was 38% and many plants died prematurely. This high level of stem canker contributed to the low seed yields and the extremely dry weather during pod fill also severely depressed yields. The mean yield of 12.3 bu/acre was the lowest recorded during this study. For the first time in three years, early season observations indicated that there was a difference in stem canker incidence due to previous crops. There appeared to be a reduction following canola and an increase following millet. In the final disease counts, these differences were not significant (P=0.05) and were not reflected in seed yield. Additional rotation cycles will be needed to determine if there is a developing influence of previous crops on stem canker incidence. Problems encountered. All major research objectives will be met. However, as anticipated at the start of the project, more data than can be determined in three years is needed. The major problems involved the demonstration plots at SunBelt Expo and the Speir farm at Plains. The plots at both sites required more on site supervision than anticipated. Shortly after the project began, strong disagreements between the management of SunBelt and College of Agriculture administration resulted in reduced activity by the University of Georgia at the Expo. Also less time available by the Extension PIs because of increased workload due to continuing reductions in state funding to the College made timely management of this site difficult. In addition, one of the Extension PIs, Boyd Padgett, left the University for another position in 1996 and his position was not refilled because of budget constraints. The Speir farm site was useful but more work needed to be performed by the Branch Station staff than anticipated. Mr. Speir attended most project meetings but could not be persuaded to try canola on his farm although a number of farmers in nearby counties have experimented with the crop on their farms. This was probably due to the fact that peanut and particularly cotton prices have been very good in the past three years and he committed most of his acreage to those crops. On farm demonstrations have educational value but obtaining reliable research data is more difficult. The only problem at the research site was the freezes during the 1995-96 season resulting in loss of the canola crop. This resulted is less disease buildup than expected and reduced the amount of data collected during the project. More data should be available by the spring of 1998. The loss of the crop showed the need for better adapted cultivars to make this an economical crop in the region. Role of Southern SARE in meeting objectives. Southern SARE has provided satisfactory assistance with administration of the budget and has been responsive to some minor adjustments that were needed including no-cost extensions. Multidisciplinary research on entire cropping systems is often difficult to facilitate. The project allowed us establish large field plots for an extended period. Small plots are much less conducive for study of plant pests which move easily from site to site. It also gave the opportunity to follow specific rotation sequences which normally are not possible. Partial funding to continue the project has been obtained and other funds are being sought. The project has been modified for the 1997-98 season to emphasize more rotations with canola in the winter and cotton will be added in the summer and millet reduced. The recent increase in cotton in the Southeast, often with continuous production, is raising the same concerns about continuous wheat:soybean this project addressed. Take-all will continue to be monitored as will Sclerotinia rot of canola and stem canker of soybeans and other diseases and both harmful and beneficial insects on all crops. Although this project focused on pest management, it also shed light on other aspects of crop management. There have been anecdotal reports of reduced plant stands in several crops following canola, including cotton. We found sound evidence of this in millet and soybean. We will investigate the effect of canola on cotton stands next year. The project has given a multi disciplinary team the support to initiate a long-term study of field crop rotation systems in the Southeast that can be expanded and bring other researchers into the project as it continues.