The hydrophobic protein cerato-ulmin (CU), produced by Ophiostoma ulmi, has been implicated in the pathogenicity of this fungus on elm. Primers were designed based on the nucleotide sequence deduced from the published CU amino-acid sequence, and a DNA fragment of the cu gene was amplified using the polymerase chain reaction. The amplified cu fragment was used as a hybridization probe to identify and isolate the cu gene from a genomic DNA library of an aggressive isolate of O. ulmi ( = O. novo-ulmi). The cu coding region is interrupted by two introns and encodes a 100 amino-acid prepro-CU polypeptide that is processed to a 75 amino-acid mature protein upon secretion. CU shows significant sequence similarity to hydrophobins secreted by certain other fungi.

Restriction fragment length polymorphisms from PCR amplified ribosomal DNAs of three Trichogramma species, T. minutum, T. brassicae, and T. near sibiricum, were studied. Length variation in the internal transcribed spacer (ITS) region was observed. The ITS region of T. brassicae is about 1350 base pairs (bp) in length and those of T. minutum and T. near sibiricum are 1300 bp. These three species also differ in the size of their ITS1 and ITS2 regions. Restriction enzyme digestions of these regions showed unique banding patterns for each species. The amplified 18S region of ribosomal DNA is about 1800 bp in length and showed no length variation between the three species of Trichogramma. Restriction enzyme digestion of this region by BamHI differentiated T. brassicae from the other two species. Restriction site maps of the ITS and 18S regions were constructed for each species. The amplified 28S region is about 1700 bp for these three species. Restriction of this region by RsaI and SacII differentiates these three species. The reported results indicate that these species of Trichogramma can be clearly differentiated from one another by nuclear ribosomal DNA markers.

Variability in virulence were detected among geographic isolates of Grypbonectria cubensis (Bruner) Hodges. Inoculation studies of isolates, originally cultured from Brazil, Africa and Hawaii, in provenance of Eucalyptuspellita F. Muell (resistant) and E. saligna Sm. (susceptible) showed that the variation in resistance of Eucalyptus spp. is quantitatively isolate-specific; the variation in virulence quantitatively host-specific.The practical consequences of pathogenic variability of C. cubensis and the strategies of selection and breeding of Eucalyptus for resistance is discussed.

A correlation between annno acid utiliz.ition and pathogenicity among five strains of Ceratocystis ulmi (Buis.) Moreau has been observed. Conditioning inoculation with a non-.iggressivo strain of C. ulmi rendered white elm seedlings resistant to further attack by an aggre.ssive strain of the same pathogen.

Little genetic information exists comparing aggressive and non-aggressive isolates of the causal agent of Dutch elm disease, Ophiostoma ulmi. Two genetic elements were compared between the subgroups. The ceratoulmin cu gene product has been associated with disease symptoms. Nucleotide-sequence analysis of cu and the internal transcribed spacer (ITS) region of the rDNA were made from three aggressive and three non-aggressive isolates of the pathogen. Our results suggested uniformity within, and unique differences between, subgroups. Differences were detected for cu in the promoter, coding, and transcription termination regions. Sequence data for the ITS clearly distinguish the subgroups.

Alcohol extraction was made from seedlings of Ulmus americana L. first inoculated with either a nonaggressive or an aggressive strain of Ophiostoma ulmi (Buisman) Nannf. and cross-inoculated 2 weeks later. The content of mansonones A,C,E,F, and G, phytoalexins in U. americana, was determined weekly for 1–6 weeks after the first inoculation. Inoculation with the nonaggressive strain led to greater levels of mansonones than did inoculation with the aggressive strain. The aggressive strain counteracted the effect of the nonaggressive strain in cross-inoculated seedlings. Mansonones F and E were the two quantitatively most important mansonones, representing 40.0 and 32.4%, respectively, whereas mansonones G,C, and A represented 15.2, 8.7, and 3.7%, respectively, of the total quantities of mansonones A,C,E,F, and G present in each sample.

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