A new canker disease, commonly known as sudden oak death, of Lithocarpus densiflorus, Quercus agrifolia, Q. kelloggii, and Q. parvula var. shrevei in California is shown to be caused by Phytophthora ramorum. The pathogen is a recently described species that previously was known only from Germany and the Netherlands on Rhododendron spp. and a Viburnum sp. This disease has reached epidemic proportions in forests along approximately 300 km of the central coast of California. The most consistent and diagnostic symptoms on trees are cankers that develop before foliage symptoms become evident. Cankers have brown or black discolored outer bark and seep dark red sap. Cankers occur on the trunk at the root crown up to 20 m above the ground, but do not enlarge below the soil line into the roots. Individual cankers are delimited by thin black lines in the inner bark and can be over 2 m in length. In L. densiflorus saplings, P. ramorum was isolated from branches as small as 5 mm in diameter. L. densiflorus and Q. agrifolia were inoculated with P. ramorum in the field and greenhouse, and symptoms similar to those of naturally infected trees developed. The pathogen was reisolated from the inoculated plants, which confirmed pathogenicity.

Tree mortality caused by Heterobasidion annosum Fr. (Bref.) in white fir (Abies concolor (Gord. & Glend.) Lindl.) often appears in clusters; symptoms in the infected trees include sapwood and heartwood decay in tree boles and roots. Although the pathogen can spread from tree to tree through root contacts, it is often confined to the initially infected trees or stumps. We devised a field inoculation study to determine comparative virulence of fungal isolates, rates and modes of fungal colonization, preferential direction of fungal colonization, and effect of root size on fungal growth in white fir roots. Fifty trees were inoculated with eight H. annosum isolates, and sampled at 4 and 12 months. Heterobasidion annosum caused purple-brown staining and incipient wood decay within 4 months. Isolates from stumps were as virulent as isolates from trees. Fungal colonization was less in smaller than larger roots. Fungal colonization was greater in the proximal (towards the bole) than in the distal direction. Most fungal colonization in the distal direction occurred during the 4 months postinoculation, while colonization in the proximal direction occurred throughout the year.Résumé : La mortalité causée par Heterobasidion annosum Fr. (Bref.) chez le sapin argenté (Abies concolor (Gord. & Glend.) Lindl.) se entraîne souvent la formation de trouées. Chez les arbres infectés, les symptômes incluent la carie du bois d'aubier et du bois de coeur dans le tronc et les racines. Même si le champignon pathogène peut se propager d'un arbre à l'autre par les greffes de racines, il est souvent confiné aux arbres ou aux souches initialement infectés. Nous avons conçu une expérience d'inoculation au champ afin de déterminer la virulence relative d'isolats du champignon, le taux et le mode de colonisation du champignon, la direction préférentielle de la colonisation et l'effet de la dimension des racines sur la croissance du champignon sur le sapin argenté. Cinquante arbres furent inoculés avec huit isolats de H. annosum et échantillonnés après 4 et 12 mois. Heterobasidion annosum a causé une coloration brun-pourpre et un début de carie en dedans de 4 mois. Les isolats qui provenaient de souches étaient aussi virulents que ceux qui avaient été récoltés sur des arbres. La colonisation était plus faible dans les petites racines que dans les grosses. Elle était plus forte en direction du tronc que vers l'extérieur. La plus grande partie de la colonisation vers l'extérieur est survenue pendant les 4 mois qui ont suivi l'inoculation, tandis que la colonisation vers le tronc se poursuivait pendant toute l'année. [Traduit par la Rédaction]

ABSTRACT.Garbelotto, M., Cobb, F. W., Bruns, T. D., Otrosina, W. J., Popenuck, T., and Slaughter, G. 1999. Genetic structure of in white fir mortality centers in California. Phytopathology 89546-554.The structure of Heterobasidion annosum populations was studied in 1.5 mixed-conifer sites in central and northern California. Study sites displayed mortality of white fir trees in enlarging discrete patches (mortality centers). At each site, fungal genotypes were defined by somatic compatibility tests. In two sites, further genetic and molecular analyses were performed on field genotypes and on homokaryons obtained by dedikaryotization of field heterokaryons. Isolates were found to be colonizing mostly the roots and the bole sapwood of white fir trees, and no significant infections of other tree species were observed. Each mortality center was characterized by the presence of several fungal genotypes, all belonging to the S intersterility group. Both homokaryotic and heterokaryotic strains were present in all sites. Multiple genotypes were retrieved in individual trees or stumps. Out of 228 fungal genotypes, 86% were found only within a single tree or stump, while 14% had spread to adjacent trees. The two largest genotypes had diameters of 9 and 10 m, and had colonized five and nine trees, stumps, or both, respectively. The maximum distance between two adjacent trees colonized by the same genotype was 6 m, and a highly significant correlation was found between tree diameter and distance of fungal "vegetative" spread. The largest clones were found in areas characterized by high tree and stump densities, and secondary spread of the fungus was more significant in denser stands. In most cases, original infection courts of existing genotypes could be traced to standing trees and not to stumps. The genetic analysis performed in two mortality centers revealed that most local genotypes had different mating alleles, and thus originated from unrelated basidiospores. In a few cases, the same mating allele was shared by two heterokaryons (n+n genome) or by a homokaryon (n genome) and a heterokaryon. Molecular analysis showed that nuclei bearing the same mating allele were identical, providing evidence that the two nuclei forming heterokaryons can act independently in the field and can be shared among isolates, presumably via di-mon mating or by separate matings of different portions of widespread homokaryons.

2 ) in 1998. The pathogen is primarily spread among trees via root contacts; therefore, the spread and impact of H. annosum from the stump(s) where infection originated (gap makers) were characterized by describing the area of potential root contact as a variable-area plot. The potential root contact zone delineated the area that would be encompassed by mortality if all trees potentially in contact with the original gap makers had died. Of the 21 plots, the actual area of tree mortality exceeded the potential root contact zone in only two gaps. Enlargement rates around gap perimeters were uneven, ranging from 0 to 1.33 m/year. Incense-cedars (Calocedrus decurrens (Torr.) Florin) tended to die more slowly than ponderosa pines (Pinus ponderosa Dougl.), often 10-30 years after adjacent pines had died. Gap enlargement was not dependent on host density, basal area, or gap-maker diameter, and tended to stabilize over time, probably because of fungal competition in root wood and lack of sufficient suitable host material along gap margins. 2 ) en 1998. Le champignon pathogène s'étend chez les arbres principalement via les contacts racinaires. Par conséquent, la progression et l'impact de H. annosum à partir de la souche où avait débuté l'infection (la cause de la trouée) ont été caractérisés en décrivant la zone potentielle de contacts racinaires comme une parcelle de superficie variable. La zone potentielle de contacts racinaires délimitait la superficie où il y aurait de la mortalité si tous les arbres potentiellement en contact avec la souche à l'origine de la trouée avaient été tués. La zone réelle de mortalité des arbres a dépassé la zone potentielle de contacts racinaires dans seulement deux des 21 trouées. Le taux d'expansion autour du périmètre des trouées était variable, allant de 0 à 1,33 m·a -1 . Les libocèdres à encens (Calocedrus decurrens (Torr.) Florin) avaient tendance à mourir plus lentement que les pins ponderosa (Pinus ponderosa Dougl.), souvent 10 à 30 ans après que les pins adjacents aient été tués. L'expansion des trouées était reliée ni à la densité des hôtes, ni à la surface terrière, ni au diamètre de la souche où avait débuté l'infection et avait tendance à se stabiliser avec le temps, probablement à cause de la compétition fongique dans le bois des racines et de l'absence d'hôtes convenables en quantité suffisante à la marge des trouées. Résumé[Traduit par la Rédaction] Rizzo et al. 1510