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Plant disease epidemics resulting from introductions of exotic fungal plant pathogens are a well known phenomenon. An associated risk - that accelerated pathogen evolution may be occurring as a consequence of genetic exchange between introduced, or introduced and resident, fungal pathogens - is largely unrecognized. This is, in part, because examples of natural, interspecific hybridization in fungi are very rare. Potential evolutionary developments range from the acquisition of new host specificities to emergence of entirely new pathogen taxa. We present evidence from cytological behavior, additive nucleotide bases in repetitive internal transcribed spacer regions of the rRNA-encoding DNA (rDNA), and amplified fragment length polymorphisms of total DNA that a new, aggressive Phytophthora pathogen of alder trees in Europe comprises a range of heteroploid-interspecific hybrids involving a Phytophthora cambivora-like species and an unknown taxon similar to Phytophthora fragariae. The hybrids' marked developmental instabilities, unusual morphological variability, and evidence for recombination in their internal transcribed spacer profiles indicates that they are of recent origin and that their evolution is continuing. The likelihood of such evolutionary events may be increasing as world trade in plants intensifies. However, routine diagnostic procedures currently in use are insufficiently sensitive to allow their detection.

A recent survey in Himachal Pradesh, Western Himalayas, India, revealed the occurrence of a previously unknown endemic Dutch elm disease Ophiostoma in scolytid breeding areas in the bark of Ulmus wallichiana trees. Ophiostoma is similar to the pathogen O. novo-ulmi which caused the recent pandemic. This discovery exposes the risks of certain pests and pathogens in forests and promotes research on the ecology and population of endemics.

The cork oak tree varieties Quercus suber and Q. ilex are suffering widespread population decline in southwest Spain and other parts of the Mediterranean. Symptoms of individual trees and their geographic distribution suggest one cause is the water- and soil-borne fungus Phytophthora. This fungus is exotic, and the damage it is causing illustrates the danger posed to forest systems by increasing intercontinental trade. Global warming could contribute to this particular pathogen's impact.

Analyzes results of pathogenicity tests on the bark of Alnus glutinosa with standard and variant hybrid alder phytophthoras. Aggressiveness on living alder logs; Levels of pathogenicity; Rates of lesion development.

The nucleotide sequences of the cerato-ulmin (cu) genes of two naturally occurring pathogenic CU-deficient mutants, PG470 and MAFf8, of the Dutch elm disease fungus, Ophiostoma novo-ulmi, were determined. The PG470 cu gene sequence was identical to that of CU-secreting isolates of O. novo-ulmi, except for a G to A mutation in the GT splice consensus sequence at the start of intron 1, suggesting that the CU deficiency was due to a splicing defect in the premRNA. In contrast, the MAFf8 cu gene showed a 99.1% sequence identity with cu genes of O. ulmi isolates, but only 92.8% sequence identity with cu genes of CU-secreting isolates of O. novo-ulmi, and in a dendrogram clustered with cu gene sequences of O. ulmi isolates with 100% bootstrap support. Restriction fragment length polymorphisms of the ribosomal RNA region, random amplified polymorphic DNA markers, and many biological properties of MAFf8, including pathogenicity, were typical of O. novo-ulmi. It is therefore likely that the cu gene of MAFf8 has been introgressed from O. ulmi, probably as a result of rare hybrid formation between O. ulmi and O. novo-ulmi, followed by backcrossing of the hybrid with O. novo-ulmi. The presence of an O. ulmi-like cu gene in MAFf8 is consistent with its CU deficiency, since the O. ulmi cu gene is known to be poorly expressed and O. ulmi isolates secrete little or no CU in culture. [ABSTRACT FROM AUTHOR]

Examines the complexity of double-stranded RNA (dsRNA) elements in an isolate of the Dutch elm disease fungus, Ophiostoma novo-ulmi. Use of high resolution polyacrylamide gel electrophoresis; Reversal of dsRNA relative mobility; Influence of structure and size on electrophoretic mobility.

A diseased isolate of Ophiostoma ulmi was found to contain 10 segments of double-stranded RNA (dsRNA) with molecular weights ranging from 2.40 x 106 to 0.23 x 106. In contrast seven other healthy isolates in the same vegetative compatibility group contained either no dsRNA or up to four dsRNA segments. Transmission of the 10 dsRNA segments. In a genetic cross in which the diseased isolate acted as the female parent single-ascospore progeny were healthy and either contained no dsRNA or only one segment of dsRNA. When elm trees were inoculated with diseased isolates, subsequent reisolations were healthy and retained only two to seven of the dsRNA segments or were diseased and retained all 10 dsRNA segments. Following conidiogenesis a diseased isolate gave rise to single-conidial progeny which were either slow growing and diseased, like their parent, with all 10 dsRNA segments, or faster growing like healthy isolates. Some of the faster growing conidial retained only two to seven of the dsRNA segments and were disease-free. However a majority of the faster growing conidial isolates retained all 10 dsRNA segments and were shown to carry disease in a latent form. The possibility that the disease of O. ulmi is conferred by specific segment of dsRNA and the potential of d-factors for the control of Dutch elm disease are discussed. [ABSTRACT FROM AUTHOR]