Crown and root rot of strawberry, caused by Macrophomina phaseolina, have become predominant soilborne diseases of strawberry in Israel over the past 5 years. In total, 151 isolates of the pathogen were isolated from infected strawberry plants of commercially grown cultivars in Israel onto a modified agar medium for the genus Macrophomina. Sclerotia viability declined more rapidly in soil maintained at 25°C or at soil temperatures fluctuating from 18 to 32°C under greenhouse conditions, compared with sclerotia viability in soil kept at 30°C. After 30 to 40 weeks of exposure in soil, inocula maintained at 25 or 30°C or at fluctuating temperatures in a greenhouse declined to negligible levels. A significant increase in plant mortality was observed in infested soils maintained at 30 versus 25°C, whereas water stress at 25 or 30°C did not affect plant mortality in M. phaseolina-infested soils. This demonstrated the importance of elevated soil temperature, not moisture stress, on plant mortality caused by M. phaseolina. Host specificity was not evident when strawberry plants were inoculated with each of seven Israeli isolates of M. phaseolina obtained from six other plant species, suggesting the importance of keeping strawberry crops out of rotation with other host crops of the pathogen. The soil fumigants methyl bromide (applied at 500 kg/ha) and metam sodium (730 liter/ha) caused 90 and 95% pathogen mortality in field experiments, respectively, indicating that fumigation may be an effective method of managing this pathogen in infested soils. The increase in prevalence of crown and root rot caused by M. phaseolina in strawberry crops in Israel may be related to the phase-out of methyl bromide.
Isolates of Colletotrichum acutatum from several hosts were characterized by various molecular methods in comparison with morphological identification. Species-specific primer analysis was reliable for grouping C. acutatum isolates to their designated species. Arbitrarily primed polymerase chain reaction and A+T-rich DNA analyses identified four subgroups within C. acutatum. Subgroup I contained U.S. isolates from almond, apple, peach, and pecan, subgroup II contained isolates from anemone, olive, and strawberry, subgroup III contained isolates from almond (Israel) and strawberry (Spain), and subgroup IV contained a single isolate from anemone (the Netherlands). Likewise, sequence analysis of the internal transcribed spacer (ITS) 2 region alone or the complete ITS (ITS 1-5.8S-ITS 2) region grouped the isolates into the same four subgroups. Percent similarity of the complete ITS region within each cluster ranged from 99.6 to 100.0, 99.8 to 100.0, and 98.6% among subgroups I, II, and III, respectively. DNA sequence analysis of the ITS 2 region alone or the entire ITS 1-2 region was more informative than that of the ITS 1 region, which could only group the isolates into two main clusters. The molecular methods employed for studying genetic variation in populations of C. acutatum determined that this species is diverse, indicating that isolates within populations of each subgroup are not host specific.
Two nonpathogenic mutant strains 4/4 and 15/15 of Fusarium oxysporum f. sp. melonis (race 1,2) were isolated by a continuous dipinoculation technique following UV mutagenesis of the virulent wild-type isolate FOM1.2. No disease symptoms or detrimental effects were observed following inoculation of muskmelon seedlings by strain 4/4. In contrast, strain 15/15 caused mortality of susceptible cultivars although to a lesser extent than the wild-type isolate. Strain 4/4 colonized a variety of muskmelon and watermelon cultivars. In muskmelon cv. Ein Dor, seedlings were dipped in a conidial suspension of strain 4/4 and planted in medium amended with the mutant to achieve 100% colonization of roots and between 30 to 70% of the lower stem tissues 7 days after planting. Similar percent colonization of watermelon seedlings by strain 4/4 was recorded. In cross-protection experiments with muskmelon cultivars, significant reduction in seedling mortality was observed between 4/4-colonized FOM1.2. challenged plants compared with that of wild-type challenged plants alone. Similarly, strain 4/4 was able to significantly reduce mortality of watermelon seedlings caused by F. oxysporum f. sp. niveum race 2. This novel approach of generating nonpathogenic mutants for biological control in Fusarium spp. and other fungal pathogens from virulent wild-type isolates may be beneficial for control, because the mutant strains, lacking only in pathogenicity, may compete more efficiently than other biocontrol organisms against the pathogen of origin.
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