PHYTOPHTHORA ROOT ROTS

HOSTS          

Phytophthora root rots have a wide host range of trees and shrubs and are found all over the world.

SYMPTOMS            

The initial symptoms are usually similar to the onset of a general decline. In conifers, the current years foliage turns yellow-green and is stunted when compared to the previous year’s needles. In successive years the entire foliage appears yellow and sparse with stunted needles in small, upturned tufts at the tips of the branches. In hardwoods, small, light green leaves are formed which produce fall coloration and drop prematurely. In successive years the foliage becomes more chlorotic and may curl and scorch during the summer.  In late stages on both conifers and hardwoods, dead branches occur in the crown and sprouts may occur on the trunk. Twig and trunk growth decrease dramatically during the disease progression. Initially, small roots are killed in increasing numbers and the brown to black lesions occur on large roots. Later, lesions occur on large roots and the buttress. Trees may decline slowly over several seasons or be killed rapidly in one or two seasons. The aboveground symptoms however, are typical of a large number of root diseases. The appearance of the foliage and growth rate of trees is similarly affected whenever the roots are infected or injured.

 DISEASE CYCLE                

Phytophthora root rots are caused by several species of fungi in the genus Phytophthora, including P. catprum. P. cinnamomi, and P. citricola. These fungi overwinter in soil as thick-walled resting spores or as vegetative mycelium in infected roots and plant debris. During the spring the spores germinate and can directly penetrate into epidermal cells or small roots or enter through wounds. Under wet soil conditions these fungi can also produce motile spores *hich are propelled by tiny flagella. These motile spores can only travel short distances in the soil but are produced in large numbers and their production, therefore, can accelerate disease development on a tree. Infected rootlets are killed in increasing numbers as more roots become infected. Larger roots are progressively infected and the tree eventually dies. Many of these pathogens also have the ability to cause cankers on the trunk and branches. During the spring, summer, and fall the fungus may produce both motile and nonmotile spores. In cold weather or extended dry periods thick-walled resting spores are produced.

 

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