The oomycete, Phytophthora infestans, is a devastating pathogen of potato worldwide. Several genotypes of P. infestans are able to infect other cultivated and weed species of the family Solanaceae and cause symptoms similar to late blight. Changes in P. infestans populations have stimulated investigations to determine if potato strains from new immigrant populations infect nonpotato hosts more often than those from the older population. Expansion of the effective host range may be one of the mechanisms involved in pathogenic changes in natural populations of P. infestans and to determine its significance, it is necessary to establish if the pathogen strains on nonpotato hosts represent distinct genotypes/populations or are freely exchanging with those on potato. This article reports characterization of P. infestans isolates from four solanaceous hosts (black nightshade, hairy nightshade, petunia, and tomato) growing within and around fields of blighted potatoes in four U.S. locations and one U.K. location and their comparison with isolates collected from adjacent infected potatoes. Isolates were characterized for mitochondrial DNA haplotype, mating type, metalaxyl resistance, allozymes of glucose-6-phosphate isomerase and peptidase, and DNA fingerprint with the RG57 probe. Analysis showed close similarity of the petunia, hairy and black nightshade isolates to potato isolates. However, tomatoes from New Jersey and Pennsylvania, respectively, were infected by two distinct and previously unreported pathogen genotoypes, which had quite different fingerprints from P. infestans isolates recovered from nearby infected potatoes. Potato growers should be aware that both weed and cultivated solanaceous species can be infected with P. infestans and may serve as clandestine reservoirs of inoculum. Because some of these plants do not show conspicuous symptoms, they may escape detection and fail to be either removed or treated and so may play a major role in the introduction and spread of pathogens to new locations.
Phytophthoracapsici, a soil-borne oomycete pathogen causing fruit rot in cucumber, has become a limiting factor for cucumber production in the Midwest. In the process of screening plant introductions (PIs) for resistance to P. capsici, it appeared that degree of susceptibility might decrease as fruits develop. To examine this more carefully, detached, greenhouse-grown, hand-pollinated `Vlaspik' fruits aged 2–18 days post-pollination (dpp) were inoculated with P. capsici mycelium and evaluated for symptoms. There was a reproducible decrease in susceptibility with increasing fruit age. The fruits that sporulated were usually younger and smaller (2–9 dpp), 10-3 dpp fruit tended to develop water-soaked symptoms, while the fruits that remained symptom-free were usually older (>14 dpp) and oversized for pickling cucumber. The transition from susceptible to more resistant appeared to occur at the end of the period of rapid fruit elongation. Detached field-grown `Straight Eight' fruits showed similar size-related trends. Candidate resistant genotypes identified from the PI screening were re-screened using 7 and 14 dpp fruits. Again an age-dependent difference in response was observed, indicating that the increase in resistance is not genotype-specific. Furthermore, field observations suggest a gradation of susceptibility within the fruits as the blossom end was most frequently infected. Preliminary tests of detached greenhouse-grown, hand-pollinated fruits suggested that as the fruits grew older, the blossom end remained susceptible longer than the stem end. These findings could have implications for appropriate screening methods, the stage of fruit likely to become infected in the field, and appropriate spray practices.
Apple (Malu ×domestica) replant disease (ARD) is a soil-borne disease syndrome of complex etiology that occurs worldwide when establishing new orchards in old fruit-growing sites. Methyl bromide (MB) has been an effective soil fumigant to control ARD, but safer alternatives to MB are needed. We evaluated soil microbial communities, tree growth, and fruit yield for three pre-plant soil treatments (compost amendment, soil treatment with a broad-spectrum fumigant, and untreated controls), and five clonal rootstocks (M7, M26, CG6210, CG30, and G16), in an apple replant site at Ithaca, N.Y. Molecular fingerprinting (PCR-DGGE) techniques were used to study soil microbial community composition of root-zone soil of the different soil treatments and rootstocks. Tree caliper, shoot growth, and yield were measured annually from 2002–04. Among the five rootstocks we compared, trees on CG6210 had the most growth and yield, while trees on M26 had the least growth and yield. Soil treatments altered soil microbial communities during the year after pre-plant treatments, and each treatment was associated with distinct microbial groups in hierarchical cluster analyses. However, those differences among fungal and bacterial communities diminished during the second year after planting, and soil fungal communities equilibrated faster than bacterial communities. Pre-plant soil treatments altered bulk-soil microbial community composition, but those shifts in soil microbial communities had no obvious correlation with tree performance. Rootstock genotypes were the dominant factor in tree performance after 3 years of observations, and different rootstocks were associated with characteristic bacterial, pseudomonad, fungal, and oomycetes communities in root-zone soil.
use because this will help reduce the risks associated with pesticide resistance. Table 1. Select fungal and oomycetes pathogens of commercial significance in the Pacific Northwest used in this study. Several studies have been undertaken in recent
Sweet basil (Ocimum basilicum) is the most economically important culinary herb in the United States. In 2007, a new disease, basil downy mildew (BDM), caused by the oomycete pathogen Peronospora belbahrii, was introduced into the United States and has since caused significant losses in commercial basil production. Although no commercial sweet basils available are resistant to P. belbahrii, other species of Ocimum have exhibited potential tolerance, resistance, or both. The objectives of this work were to determine if leaf morphological characteristics including stomata density and leaf curvature correlated with infection of plants by P. belbahrii, and thus could be used as selected characters in plant breeding. In 2011, 20 Ocimum cultivars including sweet (O. basilicum), cinnamon (O. basilicum), clove (O. basilicum), citrus (Ocimum ×africanum syn. Ocimum citriodorum), spice (Ocimum americanum syn. Ocimum canum), and holy basils (Ocimum tenuiflorum syn. Ocimum sanctum) were evaluated for susceptibility to downy mildew. Sweet basils were determined to be the most susceptible; cinnamon, clove, and Thai types were moderately susceptible; and citrus, spice, and holy types were least susceptible to downy mildew. Using those same 20 Ocimum species and cultivars, stomata length and density and leaf curvature were measured and correlated with downy mildew incidence and severity. In general, basil species with higher stomatal densities had higher downy mildew incidence and severity. High stomatal densities were mainly found in the sweet, cinnamon, and clove basils. Citrus and spice species with longer stomatal lengths generally exhibited lower downy mildew incidence. Holy basil, the least susceptible of all Ocimum sp. to P. belbahrii evaluated in this study, had the greatest stomatal density and shortest stomatal length. Some sweet basil cultivars with the highest downy mildew incidence also had the greatest downward leaf curvature, whereas other sweet basil cultivars with moderate downy mildew incidence had leaves that were nearly flat or curved upward. Holy, citrus, and spice basils with low downy mildew incidence had leaves that were nearly flat or curved upward. This study suggests that leaf curvature and stomatal density and length affect downy mildew development and sporulation. Considerations of these leaf morphological characteristics may be useful phenotypic traits in breeding for downy mildew resistance in Ocimum.
The oomycete plant pathogen, Phytophthora capsici , first described by Leon H. Leonian at the New Mexico Agricultural Research Station, is distributed globally and is a critical threat to vegetable production as a cause of damping-off, foliar
, insoluble calcium oxalate crystals could serve as herbivore feeding deterrents ( Korth et al., 2006 ; Sakai et al., 1984 ). To reduce its toxicity, taro corms and leaves are well cooked before consumption by humans. TLB, caused by the oomycete pathogen
throughout the trials. Oxathiapiprolin (Segovis) is a single-site mode of-action fungicide that suppresses oomycetes pathogens by inhibiting oxysterol-binding protein homolog, which affects lipid biosynthesis and transport ( FRAC 2021 ). Regardless of
cycles. Sporangia are formed during moist conditions, releasing zoospores that swim through water to infect plant roots, stems, and leaves. Phytophthora and other plant pathogenic oomycetes including Phytopythium and Pythium are common contaminants
in Tennessee ( USDA-NASS, 2020 ). Soilborne diseases caused by oomycetes, such as species of Pythium, Phytophthora, and Phytopythium , and fungal pathogens, including species of Fusarium, Rhizoctonia, Verticillium, and Sclerotinia, are detected