Soil may harbor fungal spores and other pathogens which are sources various plant diseases. Fungal spores are commonly transmitted onto plant surfaces by rain drops splashing off the soil surface where the spores often rest. One way to discourage this transmission and control the spread of plant fungal diseases is to use organic mulch over bare soil surfaces around plants. A layer of soft, spongy compost absorbs rain drops, and prevents any disease spores from bouncing up to nearby shrubs, vegetables or flowers. An alternative approach is to mix the compost right into the soil around plants vulnerable to fungal diseases. It turns out that other fatty acids produced in the making of compost are toxic to many fungal diseases and to certain bacterial diseases of plants. While this relationship is still very much under study, it is clear that a soil rich in compost is likely to discourage disease problems.
Some composts are "suppressive" of certain diseases; they discourage harmful root-invading fungi. Research at the Ohio Agricultural Research and Development Center (Wooster, OH 44691) has shown that pathogenic soil fungi which cause such diseases as root rots and damping-off are controlled by other soil organisms. Certain bacteria, for example, produce substances called siderophores which tie up iron, depriving the harmful organisms of this element necessary to their growth. Beneficial fungi, particularly species of trichoderma and pythium, are antagonistic to pathogenic fungi such as rhizoctonia root rot fungi and some water mold fungi. The presence of microbe rich compost in soil assures that more of these beneficial bacteria and fungi are present, therefore the probability of disease problems is diminished.
Other research has revealed that compost made the slow, passive way has some disease resistance benefits over actively managed compost which is produced more quickly. The slower, low-temperature compost appears to have better disease resisting capacity than the stuff made under high temperatures. It turns out that if you add some low-temperature compost to a batch of the high temperature stuff, the resulting material has good disease resistance.
So it may be a good idea if there is available space in the yard, if you make compost by the high-temperature method, to also start a simple pile. This slower, low-temperature compost can be mixed in with the high-temperature material whenever you add it to the soil. An alternative would be to use the compost from the outside, or cooler edge of a highly managed pile, rather than from its hot center. For example, to effect biocontrol of damping-off, a fungal disease that kills young seedlings, add lower temperature compost to their growing mix. High-temperature composting is great for killing pathogenic microorganisms, but it can kill some beneficial ones, too! By making an effort to mix in some low temperature material it just may be possible to get the best of both worlds.