Cropping intensity and organic amendments in transitional farming systems
In 2007, natural-occurring diseases, bacterial pustule and wild fire, were evaluated by percentage of leaf area infected in mid-August. These bacterial diseases were confirmed using the ooze-test and isolated on nutrient agar. The only difference is in low intensity treatment, disease severity is higher in manure-amended area (2.75% LAI) than in cover crop only area (1.25% LAI). In cover crop only area, there is no difference across three intensity treatments.
In addition to natural-occurring diseases, microplots were infested with Rhizoctonia solani and Fusarium solani inocula. R. solani and F. solani-infested areas, and a negative control (without pathogen) area were planted in randomized block design in the previously tomato variety classica planted area of 2006. Each area was 2 rows, 15 ft in length. Emergence rates were recorded one week and two weeks after planting for R. solani infested areas, as seedling rot is one of the symptoms. Three weeks after planting, 8 plants were harvested intact, and the root systems scanned for Winrhizo® analysis. Disease symptoms were recorded, including lesion length, plant height, and lesion number. Cropping history of intermediate intensity (row crops) with manure amendment has the highest number of stand. Disease rating and root characteristic showed no difference between different cropping intensity treatments in cover crop only area. There was no difference between different organic amendments in each cropping intensity. The F. solani f. sp glycine infested areas were rated for foliar symptoms at the end of August when the plants were at the R6 stage. Roots were harvested and examined for infection by sectioning the lower stem and tap root longitudinally to find gray to brown xylem vessels. No infection was found.
Soil samples were taken and assayed in the greenhouse with infestation of R. solani and F. solani on soybean. For R. solani assay, there was no significant difference (disease and root size) between organic amendments in each cropping intensity. No significant difference across cropping intensity treatments in cover crop only areas was found. For F. solani assay, the only difference was found in the intermediate cropping intensity where the root volume is higher in manure-amended area than cover crop only area. No difference was found with foliar rating in each cropping intensity treatment.
Pseudomonas population was quantified with non-culturing method, real-time polymerase chain reaction (PCR). We are still analyzing the data statistically.
Organic matter amendments to soil, in the form of preceding crop residues, cover crop residues, or direct organic matter applications, have been shown to affect levels of root and foliar diseases in several crops. Suppression of soilborne plant pathogens has been observed following additions of certain types of organic matter to soils. In some cases the mechanism of suppression in these systems has been found to be associated with increased microbial activity resulting from the influx of carbon and nitrogen supplied by the incorporated organic matter. Specific cropping systems have been shown to alter the associated soil microbial communities, and in some cases the population levels of known biological control agents have been enhanced. Foliar disease levels have been shown to be affected by applications of soil orgniac matter, even for diseases caused by pathogens that do not have a soilborne phase in their disease cycles. Possible mechanisms suggested for this type of disease suppression include changes in plant's nutrient status and the phenomenon known as systemic acquired resistance or induced systemic resistance.
Based on this information, it seemed likely that differences in the disease suppressiveness of plots in the organic transition study would vary as a result of the cropping system and organic amendment treatments. We evaluated the diseases suppressiveness of the soil to soybean root diseases in greenhouse bioassays on soil samples taken over the course of the study, and we monitored disease development on the crops in the plots to evaluate the impact of the treatments on disease development in the field. In 2007, natural-occurring diseases, bacterial pustule and wild fire, were evaluated by percentage of leaf area infected in mid-August. These bacterial diseases were confirmed using the ooze-test and isolated on nutrient agar.
Soil samples were taken and assayed in the greenhouse with infestation of R. solani and F. solani on soybean.
Pseudomonas population was quantified with non-culturing method, real-time polymerase chain reaction (PCR). The working hypothesis is field treatments had an effect on Pseudomonas population and disease suppression.
In the greenhouse bioassays, a general increase in disease suppressiveness was observed in the soils from all plots over the course of the study (2004 to 2006) for both Rhizoctonia root rot and sudden death syndrome of soybeans. However, no differences in soilborne disease suppression were detected as a result of the cropping system or amendment treatments. Differences in foliar/fruit disease levels resulting from the treatments were observed for some diseases on some crops in the field plots.
In the beginning of the transition (year 2003), no differences were seen in levels of diseases seen on tomatoes, soybeans, or pasture plants.
In 2004, levels of leaf rust on pasture grass were higher in plots receiving manure amendments, but no differences were seen in levels of wheat, cabbage, or broccoli diseases.
In 2005, higher levels on common rust were seen on corn in manure amended plots, and lower levels of powdery mildew were seen on squash plants in compost amended plots.
In 2006, the first assay year, Septoria leaf spot of tomatoes was found to be affected by both previous crop and amendment treatments, with lowest levels in the pasture plots and highest in the cash grain plots, and lower levels in the compost and manure amended plots than the non-amended plots. Incidence levels of bacterial spot of peppers were also highest in the cash grain plots and lowest in the vegetable plots.
Impacts and Contributions/Outcomes
1. Disease suppressiveness of the soils did increase over time in all treatments.
2. Level of suppression of soilborne pathogen not affected by crop system or amendments.
3. Levels of some diseases (rust) higher in manure amended plots, while others (Septoria leaf spot, anthracnose, and powdery mildew) were lower in organic amended plots.
4. Levels of effect of cropping system varied with disease, generally low in pasture and high in cash grain plots.