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Phytophthora Root Rot

CROPS IMPACTED: Tomatoes, peppers, trees, shrubs

Phytophthora Root Rot

Family: Pythiaceae

Phytophthora Root Rot Phytophthora Root Rot

About Phytophthora Root Rot

Life Cycle

There are numerous Phytophthora species that cause root rot. These soil borne pathogens mainly affect woody and herbaceous plant types such as fruit trees, ornamental trees, nut trees, and vegetable plants. Possible hosts are most susceptible to infection if they were planted too deeply in the ground or if they stay wet for a long period of time. When infection occurs in a shrub or tree, it starts at the roots and the infection works its way up to the crown. It will then move to the trunk, killing the bark inside. In favorable weather conditions, a spore only needs 4 to 8 hours to infect a host.

Phytophthora Root Rot Identification and Habitat


Plants infected with this disease will look as though they have drought stress; the plant wilts rapidly, often at the first sign of warm weather in the spring. Leaves tend to change to yellow, red, or even purple. Plants in areas where there is poor drainage are more susceptible. Unfortunately, when young trees are infected it quickly leads to death since their root system is not as robust as mature trees. In a mature tree, if only the roots are attacked, the tree can still live for a long period of time. However, the plant can die within one growing season if the base stem, or crown is infected and girdling has occurred. While symptoms vary depending on the specific pathogen species and plant type, generally infected plants will have dark areas of bark near the crown. Gum sometimes oozes from the sides of an infected trunk, and the plant will lose vigor. Red-brown strips can be found on both the inner and outer bark layers, which can be seen if some bark tissue is delicately removed. When vegetable plants are infected, roots often form spots that will turn dark brown and dry out as the infection intensifies. Different Phytophthora species can also kill young seedlings and seeds.


Phytophthora prefers weather conditions that are wet and warm. The spores created by this pathogen can survive in soil for years if there is no host to infect; however, if the soil is not moist the spores will die off within the first few months. If free water and a plant host are within close proximity to the spore, it will germinate to create motile spores which will then enter the hosts branches, roots, and crowns. The plant pathogen is easily spread by irrigation and rain, or by any equipment or soil that is contaminated with spores.

Phytophthora Root Rot Management and Control Methods

Cultural Control

An effective prevention method for Phytophthora root rot is having good soil drainage, not planting possible hosts in the ground too deeply, and choosing rootstock that is tolerant (when available). Only irrigate when necessary and avoid overhead irrigation. If trees are watered using sprinklers, use sprinkler heads with a low angle so that less of the tree trunk and branches get wet. If a drip system is used, emitters should be placed no closer than a foot to the trunk. When watering is done by hand, avoid directly watering the crown. If Phytophthora is known to be present in a certain area, think about planting new trees on 8 to 10 inch mounds. Practice good sanitation methods to avoid carrying infection to new areas of an orchard or field. If vegetable plants such as tomatoes have been infected, rotate crop type for at least one season. A resistant plant type should replace it, such as corn.

Chemical Control

When fungicides are needed for both nonbearing and bearing trees use fosetyl-al or mefenoxam. These can be applied as a foliage spray, which can absorb the fungicide and allow it to travel to the roots to help with the infection. For applications prior to planting, metam sodium or chloropicrin can also be used. With that being said, always be sure to carefully read the label for cautions and proper application before the use of any fungicide.

Alternative Phytophthora Root Rot Names

  • • Root Rot
  • • Dieback