The pathogenisis of Verticillium vascular disease was evaluated in root-inoculated yellow-poplar trees grown under favorable, periodically saturated and chronically low soil moisture levels. Verticillium was significantly more aggressive in host trees stressed by periods of low moisture than in host trees subjected to periods of flooding. At no time did inoculated, flooded trees exhibit typical Verticillium symptoms nor was pathogen reisolation obtained from the aboveground parts of these trees. In contrast, inoculated trees treated at favorable and low soil moisture levels incurred extensive pathogen colonization. Histological studies showed that the xylem vessels of petioles became occluded by fungus growth and a dark staining material at the time of first symptom expression. Additionally, macroscopic and microscopic observations of flooded trees indicated that acute injury occurred in the stem base and, under some conditions, the cortex in this region developed an aerenchyma induced by flooding.
ABSRACT In 1996 to 1998, a late-blight survey was conducted in potato- and tomato-growing regions of Morocco. A total of 149 isolates of Phytophthora infestans were collected and analyzed for the glucose-6-phosphate isomerase (Gpi) and peptidase (Pep) alleles, mating types, and metalaxyl sensitivities. Four genotypes were identified: MO-1 (mating type A1, Gpi 100/100, Pep 92/100), MO-2 (mating type A1, Gpi 86/100, Pep 92/100), MO-3 (mating type A2 Gpi 100/100, Pep 100/100), and MO-4 (mating type A1, Gpi 100/100, Pep 100/100). The potato isolates were MO-1 (1996 & 97), MO-3 (1998), and MO-4 (1998). The frequencies of A1 (MO-4) and A2 (MO-3) mating types in potato fields in 1998 were 26 and 74%, respectively. Potato isolates were pathogenic to both potatoes and tomatoes. The isolates collected from tomatoes in 1997 and 1998 were MO-2. Potato and tomato isolates were insensitive and sensitive to metalaxyl, respectively. The change of genotype population in 1998 was probably caused by migration of a new genotype from Europe associated with importation of potato seed. The detection of A1 and A2 mating types in the same potato field indicates the potential for sexual reproduction of P. infestans in Morocco.
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