Phytophthora cinnamomi as a cause of oak mortality in the state of Colima,Mexico

This research identifies the root pathogen Phytophthora cinnamomi as the pr imary cause of mortality in a 300-ha disease center of mixed oak trees in a native forest in southern Mexico. In increasing order of apparent field re sistance to the disease, the major oak species are Quercus glaucoides, Q. p eduncularis, and Q. salicifolia. P. cinnamomi was isolated from soil in the affected area from symptomatic trees and was successfully used to perform Koch's postulates on these three oak species. Artificial and natural infect ions produced vertically elongated discolorations in the outer xylem and di stinctive phloem canker lesions with a sharp demarcation line between healt hy and affected tissues. In Q, glaucoides there is little evidence that thi s oak species is able to resist the girdling effects of the phloem lesions, but in Q. peduncularis, and especially in Q. salicifolia, increased produc tion of callus tissue around the phloem canker lesions suggests an active r esistance mechanism that may allow these infected trees to survive somewhat longer. This particular incident is unlike other recent reports in other p arts of the world of oak mortality caused by P cinnamomi because the initia l appearance of disease in this area is known (just prior to 1987), and it has subsequently expanded to the present area of 300 ha (in 1999) as a dist inctive infection locus with periodically advancing infection fronts. This incident is also another dramatic illustration of the potential environment al damage that can result when P. cinnamomi is introduced into a simple for est ecosystem where the major overstory trees are susceptible to infection and are killed.