Mulch and the spread of plant pathogens
Mulch is any material placed on the soil to cover and protect it. Applying mulch can control weeds and improve plant growth by conserving soil moisture and improving soil conditions.
Certain types of plant pathogens can be spread in some types of organic mulch. Depending on the type of pathogen and which plant parts will be used, you can apply the material as mulch around hosts after you chip or grind and properly handle it (e.g., by adequately heating or thoroughly composting).
Do not apply fresh material from diseased plants under the same hosts, especially during the rainy season. If you chip and allow mulch to dry out before use, most plant-pathogenic bacteria and viruses and many fungi that infect the branches, flowers, fruit, leaves, and twigs of trees and shrubs will not spread in the mulch. Likewise, most wood decay fungi that affect the upper trunk and branches do not survive in chipped, thoroughly dried wood.
Vascular wilts. Certain vascular wilt fungi persist in plant material and soil, such as the Fusarium that infects and kills date palms. Chips from plants affected by these pathogens should not be used for mulch or allowed to enter greenwaste.
Root-decay pathogens. If trees are infected or killed by root pathogens, wood chips from the branches and upper trunk of the trees generally do not pose a risk of infection. However, bark and wood from the lower trunk, root crown, and roots (e.g., from stump grinding) can be contaminated with pathogens such as Armillaria and Phytophthora spp. These plant parts should not be used beyond the already infested area unless they are treated to eliminate the pathogens.
Wood from Armillaria-infected trees can be used as mulch after it has been chipped well or ground to avoid large (>2-inch) chunks and spread until it completely air-dries. For Phytophthora spp. and certain other pathogens, wood chips must be heated sufficiently through proper composting, solarization (e.g., in bags or containers), steam, or other means to ensure that any Phytophthora spores are killed.
Treatments. Proper treatment includes finely chopping or grinding the wood then exposing all the material to temperatures of at least 131°F for a period of 3 days or longer, depending on the particular process. See the composting regulations at calrecycle.ca.gov for the proper methods.
After proper composting or other heating methods, store the material on a clean surface and keep it covered to prevent contact with fresh debris, soil, and other untreated material that could contaminate it.
Vascular wilt fungi can persist in wood after it's chipped
Chipped wood and leaves for mulch
Applying a composted mulch