Phytophthora cinnamomi, also known as cinnamon fungus, is a soil-borne water mould[1] that produces an infection which causes a condition in plants variously called "dieback", "root rot", or (in certain Castanea species), "ink disease".

[3] It affects a range of economic plants, including food crops such as avocado and pineapple; as well as trees and woody ornamentals such as Fraser firs, shortleaf pines, loblolly pines, azaleas, camellia and boxwood, causing root rot, dieback and death of infected plants.

[7] Sexual reproduction in heterothallic Phytophthora species ordinarily occurs when gametangia of opposite mating type interact in host tissue.

Methods of transmission include local invasion via contact between the roots of infected and susceptible plants, downslope movement in surface or subsurface water such as rivers or irrigation water, zoospore dispersal over long distances via wind-blown soil and debris, and transport of infected plant matter and soil, for example via particles stuck to footwear, vehicles or equipment.

[4] However human activities such as timber harvesting, mining, bush walking, and road construction are also major methods of dispersal.

Native animals that rely on susceptible plants for survival are reduced in numbers or are eliminated from sites infested by Phytophthora dieback.

[12] Of particular concern is the infection and dieback of large areas of forest and heathland which support threatened species in the south-west Western Australia.

Damage to forests suspected to be caused by Phytophthora cinnamomi was first recorded in the United States about 200 years ago.

Infection can cause littleleaf disease of shortleaf pine (Pinus echinata), Christmas tree disease in nursery grown Fraser fir (Abies fraseri), and sudden death of a number of native tree species such as American chestnut.

Phytophthora cinnamomi is also a problem in the Mexican state of Colima, killing several native oak species and other susceptible vegetation in the surrounding woodlands.

A 1960 study of the Fallbrook, California, area correlated higher levels of avocado root rot to soils with poorer drainage and greater clay content.

[6] For specific plants such as young avocado plants, soil solarisation by using clear polythene sheets laid on the soil surface to trap radiant heat from the sun can reduce spread, and an integrated approach is generally taken to control disease on avocado.