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Anderson, R. C., D. E. Gardner, C. C. Daehler, and F. E. Meinzer. 2002. Dieback of Acacia koa in Hawai`i: Ecological and pathological characteristics of affected stands. Forest Ecology and Management 162:273-286.
ABSTRACT
Koa (Acacia koa) is an endemic Hawaiian tree that serves as a keystone species in the upper elevation forests of all the main islands. In the Mauna Loa Strip area of Hawaii Volcanoes National Park, mature koa stands are suffering from an unexplained dieback that has increased in severity since it was first noticed approximately 25 years ago. The dieback is often evident in patches, and generally spreads within stands in a radial fashion from a localized infection center. Entire crowns of affected trees become wilted, with foliage gradually progressing from an apparent healthy to a completely chlorotic condition. Although most trees die soon after the onset of symptoms, some trees are able to survive crown death by producing epicormic shoots on the lower portions of the trunk. Previously published studies reported that a vascular wilt fungus (Fusarium oxysporum f. sp. koae) was associated with koa seeds and the rhizosphere of healthy and dieback-affected koa stands. The purpose of this study was to characterize the stand structure, soil conditions, and physiological condition of dieback-affected trees, and to assess the possible role of F. oxysporum f. sp. koae in the current dieback stands. This fungus was isolated from branches of symptomatic koa in dieback-affected stands and roots from healthy and dieback-affected stands. Possible differences in the pathogenicity and virulence of F. oxysporum f. sp. koae isolates obtained from the roots of healthy koa in unaffected stands and those from branches of dieback-affected koa were determined by greenhouse inoculations of koa seedlings. Healthy koa saplings in stands unaffected by dieback were also inoculated to determine if disease symptoms could be induced by inoculation of injured roots in the field. Both branch and root isolates were pathogenic; with the percent mortality of inoculated seedlings ranging from 30 to 60% for all isolates. Disease severity between branch and root isolates was not significantly different. Wilt symptoms were not observed in field inoculations of koa saplings after 8 months. Investigation of the dieback stand structure determined that the larger size classes (i.e., DBH > 3 em) were all affected equally, with the smallest size class (i.e., 0-3 em DBH) having the lowest percentage of dieback at each site. Soil from dieback stands had higher water content and was more acidic than that from neighboring unaffected stands. Phyllodes on epicormic shoots of dieback trees had a reduced area, mass, and mass to area ratio. Phyllodes on dieback trees also had reduced stomatal conductance and higher (less negative) leaf water potential. Results from the greenhouse inoculations demonstrated that F. oxysporum f. sp. koae is pathogenic to koa, but the pathogen's activity in the field may be influenced by predisposing factors such as temperature fluctuations, water availability, soil type, and interactions with other soil organisms. F. oxysporum f. sp. koae appears to play an important role in regulating koa stand dynamics, growth, and physiology under environmental and edaphic conditions favorable for disease development.

FULL TEXT
Full text not available for this publication.

HOSTS
Acacia koa - Koa

PATHOGENS
No pathogen information available for this publication.


RELEVANT TO BIOCONTROL
No.

KEY WORDS
Acacia koa.

IMAGES
02_096 -- Rob at Koa dieback site, old weather station, Mauna Loa Strip Road 02_096 - Rob at Koa dieback site, old weather station, Mauna Loa Strip Road
PATHOGEN: Aspergillus melleus
02_097 -- koa dieback site, old weather station, Mauna Loa Strip Road 02_097 - koa dieback site, old weather station, Mauna Loa Strip Road
PATHOGEN: Aspergillus melleus
02_098 -- koa dieback site, old weather station, Mauna Loa Strip Road 02_098 - koa dieback site, old weather station, Mauna Loa Strip Road
PATHOGEN: Aspergillus melleus
02_099 -- koa dieback site, old weather station, Mauna Loa Strip Road 02_099 - koa dieback site, old weather station, Mauna Loa Strip Road
PATHOGEN: Aspergillus melleus
16_096 -- young dead Koa tree, HAVO 16_096 - young dead Koa tree, HAVO
PATHOGEN: Aspergillus melleus
27_103 -- Cindy Furukawa helping with faya tree work 27_103 - Cindy Furukawa helping with faya tree work
PATHOGEN: Aspergillus melleus
27_104 -- Lisa Nakamura at top of Mauna Loa strip road 27_104 - Lisa Nakamura at top of Mauna Loa strip road
PATHOGEN: Aspergillus melleus
28_065 -- Fusarium growing from logs from dying koa tree on Mt. Tantalus, Oahu 28_065 - Fusarium growing from logs from dying koa tree on Mt. Tantalus, Oahu
PATHOGEN: Aspergillus melleus
45_080 -- Acacia koa 45_080 - Acacia koa
PATHOGEN: Aspergillus melleus
45_081 -- Acacia koa 45_081 - Acacia koa
PATHOGEN: Aspergillus melleus
45_082 -- Acacia koa 45_082 - Acacia koa
PATHOGEN: Aspergillus melleus

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