Effects of Compost Tea and Compost Socks on Microbiological and Harvest Quality of Strawberry Fruits

Authors: Ingram, David; Millner, Patricia; Reynolds, Sara

Submitted to: International Association for Food Protection
Publication Type: Abstract Only
Publication Acceptance Date: 3/10/2009
Publication Date: 7/12/2009
Citation: Ingram, D.T., Millner, P.D., Reynolds, S.L. 2009. Effects of Compost Tea and Compost Socks on Microbiological and Harvest Quality of Strawberry Fruits. International Association for Food Protection. [abstract] . Paper No 3-32.

Interpretive Summary:

Technical Abstract: Compost tea (CT) is considered by some organic and conventional growers as a cost-effective, biologically-based control for several foliar and root diseases when used as a spray or soil drench. CT has been shown to enhance strawberry fruit yields as well as reduce disease severity of Botrytis cinerea (grey mould), a fungal fruit rot. CT may be a source of foodborne pathogens if the ingredients or the brewing processes are not properly controlled. This study determined the effects of two compost tea treatments on E. coli contamination, yield, and plant disease of strawberry fruits grown in black root-rot infested soil and compost socks. Two nutrient-supplemented aerated compost tea treatments (CT and CT amended with three yeast isolates known as biocontrols for fungal rot of fruit) and a water spray control were applied in a split-split plot treatment design to four strawberry cultivars (Sparkle, Chandler, Northeastern, Allstar) grown in Maryland in either sandy loam soil with a history of black root rot or poultry litter compost-filled socks. The CT used in this study was naturally contaminated with 2.73 cfu per ml commensal E. coli and was applied at a rate of 250ml per linear meter of plant bed (approximately 40ml per plant). Strawberry yields, percentage of diseased fruits and microbiological quality (total heterotrophs, gram negatives, total yeast, enterococci, coliforms and E. coli) were determined for seven fruit harvest events in June 2005. Compost tea treatments did not have significant effects on either harvest yield or percentage of diseased fruits when compared to water spray controls. Although each plant was thoroughly sprayed with CT, resulting in deposition of ca. 100 cfu E. coli per plant, E. coli was not detected on any fruits that matured four days post CT application. Three cultivars (Allstar, Chandler and Northeastern) produced greater yields when grown in compost socks (272.9, 146, 124.6 grams per linear meter, respectively) than in soil (148.7, 88.1, 93.7 grams per linear meter, respectively). Furthermore, cultivars grown in compost tended to have fewer (2 to 10 percent) diseased fruits than those grown in unamended soil beds, although these results were not significantly different (P greater than 0.05). Composts can be possible sources of pathogens when parts of the composted mass are inadequately exposed to lethal thermophilic temperatures or are incompletely stabilized such that sufficient nutrients are present to support pathogen regrowth. In this study, when strawberries were grown organically on stabilized, composted poultry-litter not only were berry yields enhanced, but fruit disease incidence and E. coli die-off on CT sprayed berries remained equivalent to that of berries harvested from unamended soils. These results show that growth of naturally present E. coli from low concentrations in CT sprayed directly on intact strawberry fruits and leaves is not enhanced merely by the presence of composted manure in the planting beds or by the trace nutrients in CT. Other factors, such as solar radiation exposure, dessication, microbial competition and predation, present in the plant/fruit microenvironment likely contribute substantially to die-off of E. coli on the berry fruits.