The effects of compost extract application were not consistent across all crops. Yet, some trends did emerge.
Extracts from cattle manure compost were effective in increasing marketable number and weights of
strawberries. The same extract also increased the weight of broccoli heads. Lettuce and leeks did not show any
increased harvest weights or reduced disease incidence as a result of cattle compost extract application. Crops
that received applications of chicken compost extracts had lower average harvest weights after diseased parts
were removed, except for lettuce. It is possible that the chicken compost extract had some microorganisms
associated with it or metabolites thereof that may have further contributed to disease progression or had some
other negative effect on plant growth. It is also not clear why the cattle compost extract would have had a similar
effect to water spray, unless after some storage time, the extract lost its 'potency' and had attributes more similar
to the water treatment.
Disease incidence in strawberries and broccoli did not vary significantly among plots, yet compost
applications often resulted in a greater average percentage of healthy crops compared to the control. This seems
to indicate that there was some effect either related simply to the spraying of something liquid or to something
associated with the extracts. Lettuce and leeks, however, did not show any clear response to compost extract
application. Possibly, if compost extracts are effective as a result of an induced defense reaction of the plant as
suggested by Samerski and Weltzien (1988), lettuce and leeks may be crops that are not readily induced in this
way. This study also lends support to the idea that different crops respond different to extracts from a variety of
sources (e.g. lettuce did not react to cattle extract as did the other crops).
The dilution of extracts in this study was 1:8 from the original compost; a recipe based on various
research literature (Brinton, 1995; Cronin et al, 1996). It is possible that this was too dilute for a consistent,
significant effect. In fact, researchers have found that dilutions of extracts can reduce disease inhibition
dramatically (Cronin et al., 1996; Elad and Shtienberg, 1994). Compost extract incubation time also appears to
be a variable in their effectiveness against disease. Urban and Trankner (1993) found that 24 hour extracts from
horse and cattle manure composts effectively controlled gray mold in beans. Others, however, have only found
disease suppression after an extraction period ranging from 7 to 14 days (Ketterer et al., 1992; Elad and
Shtienberg, 1994). Some researchers suggest that compost extracts lose their efficacy if they are not used within
about I week of preparation (Brinton et al, 1996). In this study, extracts were prepared in larger batches,
extracted over a I week period and were used up to 3 weeks after preparation. It is possible that then that the
mechanism responsible for inhibiting disease was much reduced as the compost extract 'aged'. This phenomenon
seems to coincide with increased berry weights early on (when the extracts were freshly prepared) with cattle
compost extract application; this was not evident later on.
There appears to be some controversy about how the extracts are prepared, anaerobically or aerobically.
Cronin and coworkers (1996), for instance, found that anaerobically prepared extracts from spent mushroom
substrate were much more effective in inhibiting apple scab than aerobically treated extracts. Weltzien (1991)
and Brinton (1995) also promote the anaerobic method of compost extract preparation. These researchers
suggest that the likely disease-suppressive effect is a result of a metabolite produced by anaerobic microorganisms
in the extract (Cronin et al., 1996). In contrast, there is also evidence that indicates that aerobically produced
compost extracts are much more effective (reviewed in Anonymous, 1996). Microbiological studies have also
shown that aerobic microbes dominate compost extracts (Sackenheim, 1993 in Hoitink et al., 1997). The method
used in this experiment was largely anaerobic with only occasional stirring during the extraction period. If,
indeed, it is an aerobic microbial population that is responsible for disease suppression, then the extracts in this
study would have been relatively ineffective.
Given that this growing season was unusually cool and wet, it is possible that populations of pathogens
were favoured and could easily outcompete any beneficial organisms associated with the extracts. High pathogen
populations could also be less impacted by inhibitory substances produced by organisms in the extracts.