Organisms Over wintering on Soil Surface
• Many organisms survive on old leaves, branches, mummied fruit, and other debris on the soil surface.
• Certain control measures are designed specifically to handle surface organisms.
• Placing a pine needle or leaf mulch beneath shrubs or between the rows in the garden forms a barrier which prevents organisms from moving from soil to plants.
• Before a new mulch is laid, all diseased debris should be removed.
• Cultivating under fruit trees destroys old, mummied fruit and prevents the organism from reproducing and infecting the new crop.
• When soil is turned four to six inches deep, organisms on the soil surface are buried so deeply that they cannot come in contact with plants.
• Removing all old leaves and stems from beneath trees and shrubs eliminates most of the disease organisms on the soil surface.
• Many diseases reproduce in dead tissue on the soil surface.
Organisms Living in the Soil
• Certain organisms live their entire life in the soil, and practically all soil contains parasitic organisms.
• Most pathogens can live in the soil from 1 to 4 years in the absence of a susceptible host.
• However, a few pathogens can live in the soil for 30 years without feeding.
• Crop rotation is a procedure in which non-host crops are used until the pathogenic organisms die out and susceptible crops can once again be grown.
• This works very well in areas where pathogens die within one to four years in the absence of a susceptible host.
• Some soil organisms attack only certain crops so these crops should not be grown in the same part of the garden each year.
• Resistant varieties are the only solution to soil organisms that can live in the soil for 20 to 30 years without a susceptible host.
• Wilt-resistant tomatoes are a good example of this kind of disease control.
• Always select disease-resistant vegetable varieties.
• Chemicals can be used to treat soil in cases where crop rotation is ineffective or when resistant varieties are not available.
Organisms Living in Dead Wood
• Several diseases which attack apples, stone fruits, grapes, and many woody landscape plants live and reproduce in dead wood.
• Pruning all diseased and dead wood will destroy a major portion of this inoculum.
• Less spraying is necessary when this source of infection has been removed.
Organisms Disseminated by Wind
• Many diseases are brought into the garden from great distances by the wind.
• The only means of controlling diseases spread in this way is to protect the foliage with chemicals.
• Since we do not know when the wind might blow spores into the garden, we should use protective chemicals on a regular basis.
When spores are blown into the garden during dry weather, they do not germinate and penetrate the tissue, so less fungicide is needed during dry weather.
• Windblown spores need a wet surface in order to germinate. For this reason, it is best not to water the garden in late afternoon, allowing the foliage to remain wet during the night.
• Some spores can penetrate wet tissue in 12 to 15 hours.
Organisms in Seeds
• Organisms can easily live in seed and are often spread from garden to garden in this way.
• For this reason, unless a unique garden variety is being preserved, gardeners should not save seeds from their garden, but should purchase seeds that were produced in parts of the country where diseases do not occur.
• Seedborne diseases can also be greatly reduced by using a chemical seed treatment.