We intended to evaluate the biological impacts of using digested dairy manure on a root crop (potatoes) under field conditions and in a greenhouse experiment to document the potential for common scab in potatoes. The major work completed for this study included a field trial with two types of potatoes and different manure applications as well as a controlled greenhouse study. Both trials produced a potato free of common scab. The results of this study will affect my operation by allowing me to distribute the digested cow manure off my farm to local potato growers, knowing that anaerobic digested manure does not cause common scab on potatoes.
Determine the biological effects of using digested cow manure as a fertilizer component on horticulture crops.
We assessed the potential of anaerobic digested dairy manure to support the development of common scab in a field in Whatcom County with a history of common scab, during the 2006 growing season. Two fresh market potato cultivars, red-skinned Chieftains and white-skinned Cal-Whites, were planted in two non-randomized strips with applications of anaerobic digested manure and digested effluent obtained from the anaerobic digester located on the Vander Haak dairy in Lynden, WA.
At harvest, 100 tubers from four locations in each of the field treatments were rated for the incidence and percent surface area covered with common scab lesions. We found no incidence of common scab on any tuber evaluated from the treatments established in 2006. Although there were no control plots (without digested dairy manure) within the field assessment, it seems that the application of digested manure did not enhance the incidence of common scab. However, since proper controls were omitted, further field studies are warranted to confirm these results.
A contributing factor to the lack of common scab within this field may be a function of the soil pH. The pH of the field soil was 4.9 where common scab is greatly suppressed in soils with a pH of 5.2 or lower. Tubers grown in acidic soil may develop scab-like lesions. This may be due to acid scab, a similar disease to common scab caused by the related pathogen S. acidiscabies. The acid scab pathogen can grow in soils with a pH as low as 4.0. Acid scab and common scab are hard to differentiate, as lesions caused by S. acidiscabies are similar, if not identical, to those caused by S. scabies.
We did observe and report the presence of the physiological condition known as “elephant hide” on both cultivars. Typically, elephant hide is caused by dry soil conditions. During the 2006 field season, rainfall for the growing period was significantly lower than normal. The occurrence of this condition could not be linked to the application of the anaerobic digested manure.
Treatments of digested manure, digested effluent and commercial fertilizer, using the potato cultivars Cal-White and Chieftain that were planted in the field study, were also used under controlled conditions in greenhouse studies using a randomized complete block design.
The source of the common scab pathogen, Streptomyces scabies, was obtained from the USDA-ARS collections of soil pathogens at Prosser, WA. We collected the digested manure and effluent from the anaerobic digester located on the Vander Haak dairy in western Washington. We used a Hale-Clipper silt loam in the pot studies obtained from the site of the 2006 field study and is a common soil cropped to potato in Whatcom County. The pH of the soil averaged 4.9 and had a field capacity of 23%. The pH of the digested manure was 8.8 with an average pH of the soil/manure mix of 5.2.
Each potato cultivar was planted in the absence of digested manure and the pathogen and in combination with three rates of digested manure and one rate of digested effluent, and each inoculated with three levels of the scab pathogen. We designed these treatments to show if the incidence of scab lesions resulted from the addition of the digested manure alone or if lesion severity increased from the combination of digested manure and high populations of the common scab pathogen.
None of the tubers grown in any treatment of the greenhouse studies exhibited the symptoms typical of common scab (lesions). This suggests that all environmental conditions necessary to express the symptoms of common scab on tubers were not met in the greenhouse trials. Soil isolations of the pathogen taken at the beginning and the end of the experiments showed that S. scabies was alive and present in the soil during the entire period of the greenhouse studies, but the pathogen did not infect the tubers resulting in tuber lesions on the potato surface. Pathogen populations were 10 times greater in pots of the Cal-White than Chieftain cultivar at the end of the trials.
The application of digested manure or effluent in absence of the pathogen resulted in an average reduction of 12% in tuber yield for both potato cultivars. The reduction in tuber weight in the absence of the pathogen may be attributed to immobilization of N or other nutrients by resident soil organisms. Soil organisms are known to successfully compete with plants for available nutrients.
Tuber yields of the potato cultivars when grown in pots containing Streptomyces scabies without digested manure or effluent decreased an average of 15% and 32% for Cal-White and 12% and 28% for Chieftain under low and high levels of pathogen presence, respectively. These results indicate that the scab pathogen, when present in high numbers, can exert significant negative impacts on potato other than the formation of lesions.
Application of 5 or 15 tons of digested manure per acre with the pathogen present showed an additive effect on yield loss. Tuber yields decreased an additional 13% to 23% in the presence of the pathogen. Yet scab lesions did not develop. The results from the greenhouse trials also showed that Cal-White was more susceptible to yield reductions caused by S. scabies than Chieftain when infested soil was supplemented with digested manure.
The scab pathogen may be negatively impacting yield in three ways:
1) It is competing with the plant for nutrients present in the soil.
2) It is feeding on sugars and other compounds released by the potato roots and is stimulated to produce and secrete an antibiotic or other toxin that is affecting plant growth and yield.
3) The pathogen is colonizing the root systems and feeding on the roots, which restricts plant growth and tuber development, resulting in poorer yields than in pathogen-free soil.
Based on our findings, other producers with digested cow manure can look toward local crop farmers as a way to distribute a portion of the manure off their farm as well as crop farmers looking to use digested cow manure as a source of fertilizer for their crops.
RECOMMENDATIONS OR NEW HYPOTHESES
We are unsure what causes the greater reduction in yield when the digested effluent was added to soil containing S. scabies compared to the non-infested soil for both Cal-White and Chieftain. It is possible that the effluent contains compounds that are beneficial to the common scab pathogen producing additional compounds that are inhibitory to root and tuber development.
We were not able to determine the influence of the observed yield decreases in the greenhouse trials with potential yield reductions in the field. We hypothesize that there would be only a minimal loss in yield in soils low or absent of the pathogen when amended with the digested manure or effluent.