Seven static compost heaps (2.5-4.6 m3) were constructed using the Indore design, in which materials are layered sandwich style with a vertical air shaft in the center. The layers consisted of twigs, wheat straw, horse dung, plant residues, a mineral layer comprised of garden soil, carbonation sludge, and basic slag and a layer of grass sod. Vegetable crop residues infected with the pathogens served as "baits" and were placed in nylon gauze bags which were then inserted into the compost heap within the layer of plant residues. The bags were placed at varying distances from the airshaft, incorporated into the heap and retrieved at different stages of the composting. Several tests were used to determine the viability of the pathogens after being composted. Perhaps the most interesting involved placing the infested compost in holes which were planted with a host plant with a high susceptibility to the pathogens. Infection of the host plant, or the appearance of resting spores on the plant, were determinants of infection. The composting process was divided into two phases, the heat phase and the maturation phase. The pathogens were tested for their response to each of these phases separately, and for the combined effect. The hear phase occurred during the first two weeks; temperatures reached 50-70 degrees C during the first 6 days and remained above 40 degrees C for the duration of the first phase. The highest temperature was reached 3-7 days after construction of the heap. The maturation phase lasted for the next five months; temperatures were below 40 degrees C.
The results showed that three factors may be involved in the eradication of the pathogens. Heat generated during the first phase is considered to be the most important and reliable indicator, although in some instances the heating phase alone was not enough to kill the pathogens, but the entire process was. The toxicity of conversion products formed mainly during the first phase (fumgitoxic volatiles have been detected in leachates and extracts from composted hadwood bark), and microbial antagonism during the entire composting process are also possible factors.
G. J. Bollen
Department of Phytopathology
Wageningen Agricultural University
P. O. Box 8025
6700 EE Wageningen