In other cases, especially where the overwintering form of the fungus or bacteria is already up in the tree, simple blowing rain is the major means of spread.
Once airborne, the spores are carried by wind and blowing rain to fresh leaves and young fruit. For instance, when spring temperatures and moisture are conducive, the fungus that incites apple scab, Venturia inaequalis, can shoot spores up into the air from a specialized organ (a pseudothecia) that overwinters on fallen leaves and fruit. Some of these plant pathogens produce fruiting bodies that can project or propel spores into the environment. Still, these fruiting bodies perform the same function that a common mushroom performs: they produce and release spores to spread into the environment, where they can land on other suitable hosts.
Some of these are not mushrooms that most of us would recognize in fact, most are too small to be visible to the naked human eye at all. If a spore has already infected tissue, the rain may provide the necessary environment to produce fruiting bodies, like mushrooms in the case of most fungi. Rain provides the proper conditions for a pathogen to proceed in its life cycle. Indeed, without water on those surfaces, most pathogens cannot long survive. Without water on those surfaces, pathogens generally cannot infect the tissues. Proper pruning should open the plant to wind and sun penetration, allowing for rapid drying of leaf, stem, and fruit surfaces. And this water almost always comes in the form of rain, though overhead irrigation and very heavy dews can also contribute. Practically speaking, nearly all economically important bacterial and fungal fruit pathogens rely on water to increase, spread, and infect (this is not true of most viral pathogens). Why is this so? The answer begins with an understanding of the pathogens that incite the diseases. Consequently, knowing that proper pruning can aid in disease management is especially important to organic growers.
The organic grower has available only a fraction of the disease-suppressive spray products that a conventional grower can use. Properly pruning fruit trees has more than one benefit for the plants and the grower, but, arguably, the most important benefit for organic growers is management of diseases. Pruning Other Fruit Trees for Disease Control.Black Rot and Other “Summer Rots” of Apples.
Pruning strategies for apples and pears are contrasted with techniques used for other stone fruits, such as plums, peaches, and cherries. It explains how to remove diseased wood and prune branches and discusses how this can control spread of disease and help prevent hosting of pathogens that cause rot. This publication explains why pruning is an especially important disease-management tool for organic tree fruit growers. Ames, NCAT Horticulture Specialist Abstract
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