Soilborne diseases are persistent problems in potato production, resulting in reductions in tuber quality and yield. Brassica rotation crops may reduce soilborne potato diseases, but how to best utilize Brassica crops in potato cropping systems has not been established. In this research, two two-year trials were established at three different sites with histories of soilborne diseases, and up to six different Brassica crops (canola, winter rapeseed, yellow and brown condiment mustards, oriental mustard, oilseed radish, and a mustard blend) and standard rotation crops (ryegrass and buckwheat) were evaluated as rotation and green manure crops. Tuber yield did not vary substantially among the rotation crops, but rotation treatments significantly affected incidence and severity of soilborne diseases at all sites. However, results were variable among sites and years. Perennial ryegrass and mustard blend rotations reduced powdery scab disease by 31–55% relative to other rotations in the only field where powdery scab was a serious problem. Mustard blend, ryegrass, and other Brassica rotations also reduced common scab, silver scurf, and black scurf at various sites, but not consistently at all sites. At one site, mustard blend and barley/ryegrass rotations reduced black scurf (by 21–58%) and common scab (by 13–34%) relative to no rotation. Overall, disease control was not correlated with biofumigation potential or rotation crop biomass production. Although both Brassica and non-Brassica rotations provided disease reduction in potato cropping systems, no single rotation crop performed consistently better than several others.
Litter decomposition was studied at two forested watersheds in east Tennessee which differed primarily in their past history of atmospheric S input. Cross Creek Watershed, located near a large coal-fired power plant, has received greater S inputs than the more remote Camp Branch Watershed. Decomposition was estimated through the measurement of forest floor respiration, litter microflora populations, litter and soil microarthropod populations, and litter nutrient status. Average forest floor respiration rates were very similar, 6.78gCOzm-2day -1 or 2472gm-2yr -t at Camp Branch and 6.86gCO2m-2day -1 or 2505 g m- 2 yr- 1 at Cross Creek. Fractional loss rates provided estimates of annual decay rates (k) of 0.35 and 0.39 for Camp Branch and Cross Creek, respectively. Litter decomposition was estimated to contribute 23% of the total CO 2 output at Camp Branch and 26% at Cross Creek, while root respiration accounts for about 43 to 46%. Bacterial and fungal populations were about equal in size at both watersheds, with bacteria averaging 100 x 10 6 g- t of litter and fungi 23 x 106 g- 1 of litter. Total numbers of arthropods averaged 34% greater at Camp Branch. Acarina populations averaged 59% higher at Camp Branch, while Collembola numbers were about equal at the two watersheds. Nutrient mobility in the litter and soil was similar at both watersheds. The order of decreasing mobility was K, Mg, Ca, S, N, and P. Litterfall nutrient concentrations were slightly higher for all elements at Cross Creek, resulting in greater litter concentrations of Ca and Mg. Litter concentrations of S and N, however, were significantly greater at Camp Branch, indicating watershed differences in the loss rates and cycling processes of these elements. There were no differences between the loss rates or litter concentrations of P, K, and Na at either site. Overall, decomposition was similar at the two watersheds. Historic S inputs do not appear to have had a major effect on decomposition rate or decomposer organisms with the possible exception of lowered arthropod populations at Cross Creek.
Abstract The use of green manures, which involves the incorporation of fresh plant material, has traditionally been primarily for the purpose of soil enrichment through the addition of plant organic matter and nutrients. However, green manures produce many changes in soil physical, chemical and biological properties, and may also result in suppression of specific plant pathogens and diseases. Green manures increase soil microbial biomass and activity, and cause distinct changes in soil microbial populations that may be partially responsible for suppression of diseases. However, green manures of different crops and cultivars may vary considerably in their activity or efficacy against different pathogens and diseases. In particular, green manures of Brassica and related crops have emerged as most effective for management of multiple plant diseases due to their biofumigation potential, which refers to the suppression of pathogens and disease through the release of volatile toxic breakdown products, as well as other unique effects on soil microbial ecology. Brassica crops and other green manures have been used to control a variety of plant pathogens, including species of Rhizoctonia , Verticillium , Sderotinia , Phythophthora , Pythium , Aphanomyces and Macrophomina , in various crop production systems, such as for the management of multiple soilborne diseases of potato, including black scurf, common scab and Verticillium wilt. Although disease reduction with green manures can be variable and provides only partial control, it is a versatile and readily-implementable additional management strategy that should be used as an important component within a larger integrated disease management program to provide improved sustainable production systems.
Soil microbiomes play crucial roles in pathogen suppression, nutrient mobilization, and maintenance of plant health. Their complexity and variability across spatial and temporal scales provide challenges for identifying common targets—microbial taxa or assemblages—for management in agricultural systems. To understand how microbiomes in potato production soils vary across growing regions and identify commonly distributed taxa among them, we compiled a continental-scale bacterial and eukaryotic amplicon dataset of over 1,300 communities with corresponding edaphic measurements from nine U.S. field sites. Field site explained most of the variance across bacterial and eukaryotic (predominantly fungal) communities, while pH and organic matter as well as nitrate, phosphate, and potassium concentrations also varied with community structure. Bacterial and eukaryotic potato soil microbiomes showed consistent phylum-level composition across locations at the continental scale, with regional-scale differences evident among genera and amplicon sequence variants (ASVs). Core community analysis identified 606 bacterial and 74 eukaryotic ASVs, which were present, but unequally distributed, across all nine field sites. Many of these core ASVs belonged to common soil genera, such as Bacillus and Mortierella, which may reveal the functional potential involved in maintaining soil health across regionally variable soil systems.
Numerous fungi and bacteria, including existing biocontrol strains with known activity against soilborne fungal pathogens as well as isolates collected from the roots and rhizosphere of tomato plants growing in the field, were tested for their efficacy in controlling Fusarium wilt of tomato. Tomato seedlings were treated with the potential biocontrol agents in the greenhouse and transplanted into pathogen-infested field soil. Organisms tested included nonpathogenic strains of Fusarium spp., Trichoderma spp., Gliocladium virens, Pseudomonas fluorescens, Burkholderia cepacia, and others. Specific nonpathogenic isolates of F. oxysporum and F. solani collected from a Fusarium wilt-suppressive soil were the most effective antagonists, providing significant and consistent disease control (50 to 80% reduction of disease incidence) in several repeated tests. These isolates also were equally effective in controlling Fusarium wilt diseases of other crops, including watermelon and muskmelon. Other organisms, including isolates of G. virens, T. hamatum, P. fluorescens, and B. cepacia, also significantly reduced Fusarium wilt compared to disease controls (30 to 65% reduction), but were not as consistently effective as the nonpathogenic Fusarium isolates. Commercially available biocontrol products containing G. virens and T. harzianum (SoilGard and RootShield, respectively) also effectively reduced disease (62 to 68% reduction) when granules were incorporated into potting medium at 0.2% (wt/vol). Several fungal and bacterial isolates collected from the roots and rhizosphere of tomato plants also significantly reduced Fusarium wilt of tomato, but were no more effective than other previously identified biocontrol strains. Combinations of antagonists, including multiple Fusarium isolates, Fusarium with bacteria, and Fusarium with other fungi, also reduced disease, but did not provide significantly better control than the nonpathogenic Fusarium antagonists alone.
Seven different 2-year rotations, consisting of barley/clover, canola, green bean, millet/rapeseed, soybean, sweet corn, and potato, all followed by potato, were assessed over 10 years (1997-2006) in a long-term cropping system trial for their effects on the development of soilborne potato diseases, tuber yield, and soil microbial communities. These same rotations were also assessed with and without the addition of a fall cover crop of no-tilled winter rye (except for barley/clover, for which underseeded ryegrass was substituted for clover) over a 4-year period. Canola and rapeseed rotations consistently reduced the severity of Rhizoctonia canker, black scurf, and common scab (18 to 38% reduction), and canola rotations resulted in higher tuber yields than continuous potato or barley/clover (6.8 to 8.2% higher). Addition of the winter rye cover crop further reduced black scurf and common scab (average 12.5 and 7.2% reduction, respectively) across all rotations. The combined effect of a canola or rapeseed rotation and winter rye cover crop reduced disease severity by 35 to 41% for black scurf and 20 to 33% for common scab relative to continuous potato with no cover crop. Verticillium wilt became a prominent disease problem only after four full rotation cycles, with high disease levels in all plots; however, incidence was lowest in barley rotations. Barley/clover and rapeseed rotations resulted in the highest soil bacterial populations and microbial activity, and all rotations had distinct effects on soil microbial community characteristics. Addition of a cover crop also resulted in increases in bacterial populations and microbial activity and had significant effects on soil microbial characteristics, in addition to slightly improving tuber yield (4% increase). Thus, in addition to positive effects in reducing erosion and improving soil quality, effective crop rotations in conjunction with planting cover crops can provide improved control of soilborne diseases. However, this study also demonstrated limitations with 2-year rotations in general, because all rotations resulted in increasing levels of common scab and Verticillium wilt over time.
