Abstract:Phytophthora root rot on Abies fraseri trees was monitored from 2001 to 2007 within the disease front of a 12-year-old Virginia plantation where trees had been dying of the disease since 1994. After a slow increase in early foliage symptom development from July 2001 to September 2002, the frequency of A. fraseri trees with early symptoms accelerated for about 15 months. While the slow increase occurred during a 18.7% lower than normal rainfall period and the acceleration occurred during a 31.2% higher than normal rainfall period, the percentage of trees with early symptoms continued to increase during the mid-winter months (December–February) when the estimated mean minimum daily soil temperature (25 cm depth) was unfavourable (<10°C) to Phytophthora cinnamomi pathogenic activity. The time required for trees to progress from early foliage symptoms to completely dead foliage, from November 2000 to October 2007, was highly variable, ranging from 4 to 35 months. Root recovery rates for P. cinnamomi, assayed on a selective medium, were 6.4 times greater for symptomatic foliage trees than for asymptomatic foliage trees in this deep, silt-loam soil. Following an atypical cold period in February 2007, when the mean minimum daily soil temperature was 0.8°C, symptomatic roots yielded only a low level of germinable propagules of P. cinnamomi. Further, during an atypical midsummer in 2007 (June–August), when the soil water potential was at or below −9 bars for 68 of 92 days, symptomatic roots yielded no germinable propagules of P. cinnamomi. Addition of thiophanate-methyl to the selective medium aided P. cinnamomi isolation by inhibiting many undesired pythiaceous colonies growing from symptomatic roots.
Abstract: Transcriptional changes during asexual sporangia formation by the late blight pathogen Phytophthora infestans were identified using microarrays representing 15,646 genes and RNA from sporulation time-courses, purified spores, and sporulation-defective strains. Results were confirmed by reverse transcription-polymerase chain reaction analyses of sporulation on artificial media and infected tomato. During sporulation, about 12% of genes were up-regulated compared to vegetative hyphae and 5% were down-regulated. The most prevalent induced genes had functions in signal transduction, flagella assembly, cellular organization, metabolism, and molecular or vesicular transport. Distinct patterns of expression were discerned based on the kinetics of mRNA induction and their persistence in sporangia. For example, most flagella-associated transcripts were induced very early in sporulation and maintained in sporangia, while many participants in metabolism or small molecule transport were also induced early but had low levels in sporangia. Data from this study are a resource for understanding sporogenesis, which is critical to the pathogenic success of P. infestans and other oomycetes.
Abstract: A series of inbreeding crosses, recurrent backcrosses and successive sibling crosses were completed up to the sixth generation in the plant pathogen Phytophthora capsici, generating a total of 692 oospore-derived isolates. All of the crosses stemmed from an initial mating between two wild-type P. capsici isolates. The heterozygosity level, as measured through the inheritance of 20 amplified fragment length polymorphism (AFLP) markers, decreased incrementally with continued inbreeding and was reduced by ~60–75% by the second consecutive sibling cross. Of the eight crosses analysed, all but one cross produced a proportion of oospore-derived progeny that were identical to one or other parent, indicating that apomixis can play a role in P. capsici intraspecific crosses. There was no evidence of isolates emerging through selfing or generation of homothallic isolates. Overall, these results indicate that back and sibling crosses are possible and that the large reservoirs of naturally occurring genetic variation in P. capsici may be useful for developing inbred lines to characterize complex genetic traits in Phytophthora.
Abstract: Infection of hairy nightshade (Solanum sarrachoides Sendt.) by Phytophthora infestans has been reported. However, the epidemiological significance of hairy nightshade to potato late blight is not well known. Disease development and infection rates of P. infestans were quantified on hairy nightshade relative to tomato (cv. Brandywine) and potato (cv. Shepody) hosts to evaluate infection potential at 14, 18, 22 and 26°C and 72, 82, 87, and 92% relative humidity (RH). The susceptibility of hairy nightshade to inoculum levels, weed ontogeny, and sporangia production potential were also investigated. Late blight development varied among hairy nightshade, tomato and potato hosts. Pathogen infection rates ranged from 0.0325 to 0.4674 gompits/day (unit for quantifying infection rates), and were significantly (P < 0.05) greater on potato and tomato than on hairy nightshade. Late blight severity was variably affected by RH. Disease levels on hairy nightshade varied with inoculum load; and ranged from 9 to 26% and 26 to 37% at low (5 × 103) and high (25 × 103) sporangia concentrations, respectively. Late blight was recorded irrespective of hairy nightshade ontogeny, and was significantly greater on 8–10 than 4–6-week-old plants. These results indicate that pathogen, environmental and host factors affect late blight development on hairy nightshade.
Abstract: Isolates from avocado tree cankers have been recognized as a distinct subgroup within the P. citricola complex since 1974, both morphologically and molecularly (isozyme and amplified fragment length polymorphism [AFLP] analyses). This subgroup is formally separated from P. citricola after comparative DNA fingerprinting and sequence analyses of the ITS region, as well as by morphological examinations. This new taxon is homothallic, produces plerotic oospores with paragynous antheridia and noncaducous semipapillate sporangia. Morphologically it differs from other species of Waterhouse group III by producing many large bizarre-shaped sporangia and smaller oogonia with asymmetric capitate antheridia. It belongs to clade 2 and is phylogenetically closer to P. siskiyouensis, P. capsici and P. tropicalis than to P. citricola. P. mengei can be easily differentiated from its relatives in the same clade and other species of this morpho-group by DNA fingerprints and sequence analysis. This new taxon is named Phytophthora mengei sp. nov.
Abstract: Growth, sporulation, and survival of Phytophthora infestans on volunteer potato tubers, was investigated under temperatures representative of winter (4°C, 7°C, and 10°C) and spring (13°C, 16°C, 19°C) soil conditions in western Washington. Inoculated tubers stored at 10°C for 8 days had a significantly (P < 0.05) higher percentage of disease symptoms on tuber surfaces and a higher number of lenticels and eyes with P. infestans sporulation compared to those stored at 4°C or 7°C. Sporulation of P. infestans on cut tuber surfaces was observed following 3-week storage at the three winter soil temperatures. After 12-week storage, tubers inoculated with a US-8 isolate had a significantly higher percentage of late blight on cut surfaces than those inoculated with a US-11 isolate (70% versus 50%, respectively). For spring soil temperature studies, tubers inoculated with the US-8 isolate and held at 19°C had a significantly higher number of lenticels per tuber with P. infestans sporulation than tubers held at 13°C or 16°C. Sporulation of P. infestans on tuber surfaces was detected on infected tubers buried 5-cm deep in potting medium at all tested winter and spring temperatures for 3- or 6-day periods, respectively. The site or depth of tuber inoculation with P. infestans did not influence tuber-to-sprout infection events and whether apical end or stem sprouts become infected. Tubers with late blight that survive the winter in western Washington and support sporulation of P. infestans via lenticels and eyes may enable the transmission of P. infestans from infected tissues to sprouts of volunteer plants. However, the impact of these events on primary inoculum production by P. infestans in the region is probably limited by the mild, winter conditions favoring tuber break-down in soil.
Abstract: Dieback and mortality of Alnus incana subsp. tenuifolia in the Southern Rocky Mountains apparently began by the late 1980s and have become a concern to land managers. A survey of alder including 68 transects from southern Wyoming to northern New Mexico indicated that, of 6,503 standing stems, 37% were dead, 29% had dieback, and 34% were healthy. Transects intercepted 1,479 m of live and 1,177 m of dead alder canopy. A second, more localized survey with 32 transects in the upper Gunnison River watershed of Colorado yielded similar results. Abundance of live sprouts was inversely related to amount of dieback and mortality in a genet, suggesting that affected genets are dying and not replacing themselves. Damage did not vary substantially by geographic area and was not related to elevation, animal browsing, or distance to nearest road. Distance to nearest stream was weakly, inversely related to severity of dieback and mortality. Symptoms were not consistent with disease of alder caused by Phytophthora alni in Europe, and isolations for Phytophthora species were negative. Cytospora canker, caused by Valsa melanodiscus (anamorph Cytospora umbrina), is the proximate cause of the dieback and mortality.
Abstract: During the past decade, and in particular after the wet year 2002 and the dry year 2003, an increasing number of trees and stands of European beech (Fagus sylvatica L.) in Bavaria were showing symptoms typical for Phytophthora diseases: increased transparency and crown dieback, small-sized and often yellowish foliage, root and collar rot and aerial bleeding cankers up to stem heights of >20 m. Between 2003 and 2007 134 mature beech stands on a broad range of geological substrates were surveyed, and collar rot and aerial bleeding cankers were found in 116 (86.6%) stands. In most stands the majority of beech trees were declining and scattered or clustered mortality occurred. Bark and soil samples were taken from 314 trees in 112 stands, and 11 Phytophthora species were recovered from 253 trees (80.6%) in 104 stands (92.9%). The most frequent species were P. citricola, P. cambivora and P. cactorum. Primary Phytophthora lesions were soon infected by a series of secondary bark pathogens, including Nectria coccinea, and wood decay fungi. In addition, infected trees were often attacked by several bark and wood boring insects leading to rapid mortality. Bark necroses were examined for their probable age in order to determine whether the onset of the current Phytophthora epidemic was correlated to rainfall rates recorded at 22 Bavarian forest ecosystem monitoring stations. A small-scale survey in nine Bavarian nurseries demonstrated regular infestations of all beech fields with the same range of Phytophthora species. The results indicate that (1) Phytophthora species are regularly associated with beech decline and may also be involved in the complex of Beech Bark Disease, (2) excessive rainfalls and droughts are triggering the disease, and (3) widespread Phytophthora infestations of nursery stock might endanger current and future silvicultural projects aiming on the replacement of non-natural conifer stands by beech dominated mixed stands.
Abstract: Sudden oak death (SOD), caused by the recently discovered non-native invasive pathogen, Phytophthora ramorum, has already killed tens of thousands of native coast live oak and tanoak trees in California. Little is known of potential short and long term impacts of this novel plant–pathogen interaction on forest structure and composition. Coast live oak (Quercus agrifolia) and bay laurel (Umbellularia californica) form mixed-evergreen forests along the northern California coast. This study measured tree mortality over a
gradient of disease in three time periods. Direct measurements of current mortality were taken during 2004, representing a point-in-time estimate of present and ongoing mortality. Past stand conditions, c. 1994, were estimated using a stand reconstruction technique. Future stand conditions, c. 2014, were
calculated by assuming that, given a lack of host resistance, live trees showing signs of the disease in 2004 would die. Results indicate that coast live oaks died at a rate of 4.4–5.5% year-1 between 1994 and 2004 in highly impacted sites, compared with a background rate of 0.49% year-1, a ten-fold increase in mortality. From 2004 to 2014, mortality rates in the same sites were 0.8–2.6% year-1. Over the entire period, in highly impacted sites, a 59–70% loss of coast live oak basal area was predicted, and coast live oak decreased from 60% to 40% of total stand basal area, while bay laurel increased from 22% to 37%. Future stand structures will likely have greater proportions of bay laurel relative to coast live oak.
Abstract: Potato fields in Denmark, Finland, Norway and Sweden were sampled for single-lesion isolates of Phytophthora infestans. The aggressiveness of the isolates was determined on detached leaflets of potato cvs Bintje (susceptible) and Matilda (moderately resistant). The aggressiveness tests were carried out in the respective home countries of the isolates, with the exception of the Danish isolates. Fifteen Danish isolates were studied in each of the other three countries, including five isolates tested in all three laboratories. Results obtained from the Danish isolates revealed substantial differences between the test laboratories for infection efficiency, lesion growth rate and sporulation capacity on detached leaflets. When the laboratory effect was taken into account, the differences in aggressiveness between the countries were generally small or inconsistent between the test cultivars and epidemiologically insignificant. By contrast, variation among isolates within countries was substantial. The magnitude of the variation depended on country and cultivar. Maximal variation for the means of the isolates was between 89 and 185 h for latent period, between 100 and 1297 sporangia mm-2 for sporulation capacity and between nearly zero and 6 mm day-1 for lesion growth rate. Typically less than 1% of sporangia were able to cause infections, except in Norway. These extraordinarily low values may be an artefact of the testing method. High variation in results between the test laboratories emphasizes the need for caution when comparing results obtained by different research groups.
Abstract