Search

(1993). Identification of Armillaria Species from New Hampshire. Mycologia: Vol. 85, No. 3, pp. 365-368.

Diploid-haploid pairings to identify field isolates of Armillaria to species generally rely on morphological differences between diploid and haploid colonies (flat and fluffy colonies, respectively) and morphological changes in haploid tester colonies. Presumably, the change in morphology of the haploid tester is due to diploidization. Isozyme markers, aconitase and glucosephosphate isomerase, were used to follow nuclear migration from putatively diploid isolates into a haploid tester strain of A. ostoyae. After pairing diploid field isolates with a haploid tester for 3 or 4 weeks, subcultures were taken from the haploid tester at 2 and 10 mm from the confrontation zone. Subcultures from 2 mm were mostly flat; subcultures from 10 mm were fluffy, flat, or fluffy with flat sectors. Hyphal tip cultures from the subcultures were analyzed for the isozyme markers. All fluffy hyphal tip cultures had the isozyme phenotype of the haploid tester. Flat hyphal tip cultures either had the isozyme phenotype of the diploid parent or had a hybrid isozyme phenotype with markers of both the diploid parent and the haploid tester. The data suggest that in conspecific diploid-haploid pairings, the diploid nucleus usually migrates into the haploid mycelium and eventually displaces the haploid nucleus.

Armillaria rhizomorphs consist of differentiated hyphae with a melanized outer cortex. Melanin is known to prevent lysis of fungal structures by hydrolytic enzymes and may protect against antagonistic microorganisms. Our studies indicate that melanized rhizomorphs also adsorb high concentrations of cations from the surrounding soil environment. Rhizomorphs of four Armillaria species (A. ostoyae (Romagn.) Herink, A. calvescens Bérubé & Dessureault, A. gemina Bérubé & Dessureault, and A. sinapina Bérubé & Dessureault) collected from Minnesota, New Hampshire, and Oregon had substantially elevated levels of metal ions compared with the soil from which they were collected. With some elements, ions were 50–100 times more concentrated on rhizomorphs than in soil. Concentrations of Al, Zn, Fe, Cu, and Pb on rhizomorphs ranged up to 3440, 1930, 1890, 15, and 680 μg/g, respectively. Energy dispersive X-ray microanalysis showed that metal ions were located only on the outer portions of the rhizomorphs and not concentrated in the interior. A coating of metal ions may play a key role in the longevity and survival of rhizomorphs in soil. Key words: Armillaria, rhizomorphs, metals, fungal antagonism.

Armillaria root disease (causal agent Armillaria mellea sensu stricto) is a major disease of fruit and nut crops in California, USA. An aspect of Armillaria root disease in orchard crops that has not been extensively studied is the spatial distribution of the fungus on the roots of mature trees and how this translates to symptom expression and yield loss. Root systems of Bartlett pear trees at two commercial orchards were excavated utilizing one of two methods: pressurized water or supersonic air. At both sites, trees were selected to represent a range of crown conditions (from declining to apparently healthy) in and around Armillaria infection centers. Root systems were excavated to a minimum diameter of 2 m and a minimum depth of 1 m. At both sites, numerous independent infections by Armillaria were observed on the roots of diseased trees. Discrete lesions were scattered on root surfaces, ranging in size from several square centimeters to large lesions covering hundreds of square centimeters. The root system excavations revealed a lack of root contact or root grafts between adjacent pear trees. Rhizomorphs were prevalent on root surfaces and in the soil at both sites. Both methods resulted in loss of very fine roots (