Ohia decline: the role of Phytophthora Cinnamomi, CPSU/UH Technical Report No.12

Author:

Hwang, S. C.

Title:

Ohia decline: the role of Phytophthora Cinnamomi, CPSU/UH Technical Report No.12

Periodical:

CPSU/UH Technical Report #12

Year:

1977

Pages:

77p.

Subject:

Ohia dieback Soils analysis

Summary:

The population of Phytophthora cinnamomi in soils and the amount of ohia rootlets infested with this fungus were determined at four locations, each with healthy and decline forest located close together. The fungus was detected in 34% of soil samples collected from healthy areas and 29% of those from decline areas. Average population of P. cinnamomi in healthy areas (0.1-1.7 propagules/g s o i l ) was not significantly different from that in decline areas (0.1 2.2 propagules/g soil). About 37% of declining trees and also 37% of healthy trees had rootlets infested with P. cinnamomi. Declining trees had an average of 5.2% rootlet segments infested with P. cinnamomi, and healthy trees had 9.4% with this fungus. For a total of 199 ohia trees surveyed, there was no correlation between percentage of rootlets infested with P. cinnamomi and severity of tree decline. There was no indication that P. cinnamomi in decline areas was more pathogenic than that in healthy areas. These results suggest that P. cinnamomi is not a major cause of ohia decline. By contrast, in avocado tree decline caused by P. cinnamomi, the population of 1. cinnamomi in soil collected from the root zone of declining trees was significantly higher than that collected from healthy trees. The fungus was isolated from 97% of the declining trees. Moreover, the severity of tree decline was directly correlated with the percentage of roots infested with P. cinnamomi. Declining trees had an average of 29.2% of root segments infested with P. cinnamomi while healthy trees had only 8.7% with this fungus. Colonies of P. cinnamomi recovered from natural soil originated mainly from chlamydospores, and occasionally from zoospores. Chlamydospores occurred as free spores or imbedded in organic matter. Results of this study indicated that sporangia of P. cinnamomi also existed in natural soil, and zoospores found on the isolation plates were released from sporangia during incubation. Among the three spore types of P. cinnamomi tested, chlamydospores were the most persistent in soil, sporangia were intermediate, while zoospores were the least persistent. Survival of P. cinnamomi in soil was better under moist than submerged conditions. The population of chlamydospores remained detectable for one year in moist soil, while only for 3 months in submerged soil. Similarly, over a 12-month period the population of P. cinnamomi in a naturally infested avocado soil declined faster under submerged than moist conditions. Results of survival studies also showed that P. cinnamomi in the root tissue was more persistent than as free chlamydospores in soil. Under moist conditions, the percentage of root tissues from which the fungus was recovered declined only slightly after one year of incubation, while the population of chlamydospores in soil decreased to undetectable level in the same period. Phytophthora cinnamomi is a good saprophyte. It was able to colonize about 52% of ohia stem segments at a population as low as 10 chlamydospores/g of soil. Among the three spore types of P. cinnamomi tested, chlamydospores were also the most effective in colonizing dead ohia stems, while colonization potential of motile and encysted zoospores was about the same.

URL:

http://hdl.handle.net/10125/2932

Collection:

Monographs