Survival of detached sporangia ofPhytophthora infestans exposed to ambient, relatively dry atmospheric conditions

The effect of duration of exposure, daily weather conditions, and exposure to direct or indirect sunlight on survival of sporangia ofPhytophthora infestans under ambient, relatively dry atmospheric conditions was evaluated. Viability of sporangia was assessed by determining the proportion of potato tuber slices or leaflets that became infected after inoculation with exposed sporangia. The maximum survival time of sporangia was 24 h under moderately cool (mean of 15 C) and dry conditions (RH < 25%). Sky conditions were sunny during daylight hours during this exposure. Infection seldom occurred when sporangia were exposed directly to sunlight; only 25 of 566 (4.4%) groups of sporangia caused infection. Of these 25 groups, 23 (92%) had been exposed to mean temperatures below 33 C for 4 or fewer hours. Binary logistic regression analysis of the data showed that duration of exposure, direct sunlight, and type of day were all significant factors affecting survival of sporangia. Long exposure in direct sunlight limited survival. Survival of sporangia was more likely to occur on cool, cloudy days and cool, rainy days than on other day-types.     

Effect of inoculum concentration ofPhytophthora infestans on potato late blight

Zoospore suspensions ofPhytophthora infestans applied to potato cultivars in the field with an exponential inoculum sprayer resulted in defoliation within a range of severity that was log-normal with respect to inoculum concentration. Regression lines for different cultivars differed in position and, depending upon moisture conditions, slope, indicating that the cultivars differed in resistance to penetration and to invasion of tissue by the pathogen.     

Wild tuber-bearing species ofSolanum and incidence ofPhytophthora infestans (Mont.) de Bary on the Western slopes of the volcano Nevado de Toluca. 2. Distribution ofPhytophthora infestans

Summary An area of 15 000 ha on the Western slopes of the volcano Nevado de Toluca was surveyed during 1982–1986. The occurrence of late blight (Phytophthora infestans) was studied in 1986 along five long (7–10 km) transects from an altitude of 2900 m to 3900 m, the upper limit ofSolanum demissum (dms) and in all years in some other sites. Late blight was confined to scattered pockets, 100–500 m in diameter, separated by several kilometers. In 789 colonies of wildSolanum spp., each consisting of 4 to >100 plants per colony ofdms, verrucosum (ver), iopetalum, brachycarpum, × edinense, stoloniferum or one unidentified species, symptoms were found in 10% of the colonies or less than 1% of the plants in each colony. Plants had few (1–2) lesions which did not affect vigour. No infected tubers were found. Most (64%) of the infected colonies were ofdms, the predominant species (69%). Colonies ofver were less frequent (11%) and 17% were infected. No infected plants were found in the unidentified species, thought to bedms×ver.     

Occurrence of the A2 mating type ofPhytophthora infestans in India

Summary One hundred and four isolates ofPhytophthora infestans were collected during 1990–93 from different potato growing regions of India. They were paired with mating type A1 ofP. capsici to determine their mating type. All isolates from the Indo-Gangetic plains and Assam were the A1 mating type, whereas a mixed population of A1 and A2 types occurred at Shillong, Darjeeling and Shimla hills. The frequency of A2 isolates was 5–30% in the Shimla hills and 60% and 58% respectively in the Shillong and Darjeeling hills.     

Wild tuber-bearing species ofSolanum and incidence ofPhytophthora infestans (Mont.) de Bary on the Western slopes of the volcano Nevado de Toluca. 3. Physiological races ofPhytophthora infestans

Summary A survey of races ofPhytophthora infestans was made in 1985 and 1986 on the western slopes of the volcano Nevado de Toluca, Mexico. Isolates were collected from infected plants of wildSolanum species found in five long (7–10 km) and some short (300–500 m) transects. The study are of 15 000 ha was between 2900 and 3900 m altitude. Isolates were collected fromSolanum demissum (35 isolates),verrucosum (11),iopetalum (2),brachycarpum (7),stoloniferum (2), r-genotype (9) and from differential hosts (11). No isolate was identified as race 0. Ten virulence genes were identified with a set of differential hosts and the most common were numbers 2 and 4. Isolates with eight virulence genes were the most common and those with ten were found on eight occasions in the natural habitat and on three in the experimental fields.     

Incidence of the A2 mating type ofPhytophthora infestans on potato crops in Northern Ireland

Summary Of 250 isolates ofPhytophthora infestans obtained from Northern Ireland potato crops between 1981 and 1993, only six proved to be of the A2 mating type, the remainder being A1. The first A2 isolate was obtained from a tuber in 1987. The frequency of A2 isolates from 1987 onwards was 3%, a lower incidence than has been reported for England and Wales and the Republic of Ireland. There was no association between phenylamide resistance and mating type.     

Metalaxyl and other fungicides for control ofPhytophthora infestans on potato in northwest Washington State

In northwest Washington State, fungicidal control of late blight caused byPhytophthora infestons was evaluated for three years. During 1977, a high rainfall year with supplemental sprinkler irrigations, all metalaxyl and chlorothalonil treatments reduced tuber rot at harvest, but only metalaxyl extended control of tuber rot after 4 months’ storage. During 1978, a high rainfall year, one application of metalaxyl sprayed over or dusted on potato seed pieces at planting or applied five times to foliage at 2 or 4 wk intervals gave excellent blight control equal to that with chlorothalonil applied to foliage every 2 wk. In 1979, a low rainfall year,P. infestons did not kill plants in untreated plots until October. Five applications of metalaxyl sprayed every 2 wk on foliage did not control foliage blight. However, metalaxyl reduced foliage blight when dusted on cut potato seed pieces. Mancozeb sprayed every 2 wk also gave excellent control. The possibility that metalaxyl applied to foliage is effective in late blight control only when washed into the soil by rainfall or irrigation for uptake by roots is discussed. Yields and percent U.S. No. 1 grade tubers were not significantly affected by any treatment in these 3-year trials.     

Survey ofPhytophthora infestans populations in Serbia: Mating type, metalaxyl resistance and virulence properties

Summary A total of 37 single-lesion isolates ofPhytophthora infestans were collected during 2000 from different potato growing regions in the central part of Serbia. They were paired in Al and A2 matings to determine their mating types. Of these 37 isolates, 28 were found to belong to the A2 mating type, eight to the Al type and one was found to be self-fertile (A1/A2). Using the in vitro mycelial radial growth method no metalaxyl resistantP. infestans isolate could be found. The rate of occurrence of intermediate isolates was not high, on the average 2.70%. All other isolates were sensitive to metalaxyl. There was no correlation between mating type and the intermediate metalaxyl resistance found.     

Identification of mating types and metalaxyl resistance in North American populations ofPhytophthora infestans

The A² mating type ofPhytophthora infestans was first reported in the United States in 1990. Concurrently,P. infestans strains resistant to metalaxyl ere found in the Pacific Northwest. Collaborative surveys were undertaken during 1991–1993 to investigate the frequency of occurrence of A² mating types and metalaxyl resistant strains in populations ofP. infestans isolated from outbreaks of late blight in potato and tomato crops in North America.In vitro testing indicated that isolates from the northeastern U.S. and Atlantic Canada were primarily (52/55) metalaxyl sensitive and all were A¹ mating types. Among 85 isolates from late blight epidemics in Florida and Texas, greater than 61% were both metalaxyl resistant and A² mating type. Metalaxyl resistance and A² mating types were identified also in a few tomato isolates from North Carolina. Although the majority of 134 isolates from the Pacific Northwest (British Columbia and Washington) were metalaxyl resistant, only 2 isolates from Washington were A² mating types. Among 111 isolates from 2 sites in central Mexico, 63% and 77% were both metalaxyl resistant and A² mating types. The data indicate also a higher frequency of metalaxyl resistance in A² isolates, than in A¹ isolates, among isolates from Florida and Texas. Highest metalaxyl resistance levels were found, however, in A¹ isolates from California, where no A² isolates were recovered.     

Effects of hot and cold temperatures on the survival of oospores produced by United States strains ofPhytophthora infestans

Oospores produced from matings between US strains ofPhytophthora infestans were subjected to hot or cold temperatures and then evaluated for viability, based on percent germinationin vitro. Of 35 crosses, 3 produced oospores with sufficient germination for replicated assessments of temperature effects. Germination rates were as high as 26%, but the maximum germination percentage varied among crosses. Two additional crosses, one involving isolates from Mexico also had sufficient germination for assessment of temperature effects. Oospores did not germinate after 2 hr treatments at or above 46C, or after 12 hr treatments at 40C. Temperature treatment at -80C suppressed germination. Temperature had a similar effect on oospore germination from all crosses.     

Assessment of mating types and resistance to metalaxyl of Canadian populations ofPhytophthora infestans in 1997

The distribution of A1 and A2 mating types, and of metalaxyl-sensitive (MS), metalaxyl-intermediate (MI), and metalaxyl-resistant (MR) strains ofPhytophthora infestans, the causal pathogen of late blight, were determined in Canada in 1997. Potato plants or tubers and tomato plants or fruits, suspected to have late blight infection, were received from seven Canadian provinces in 1997, and were assessed for the presence ofP. infestans. Among 220 samples suspected to have late blight,P. infestans was recovered from 132 samples (60%) received from British Columbia (BC), Manitoba, New Brunswick, Prince Edward Island, and Quebec. From the 132 samples, 364 isolates were obtained and tested for their mating type and response to metalaxyl. Populations ofP. infestans within Canadian provinces were homogeneous for mating types, except within BC, where populations comprised both A1 and A2 mating types both within fields and sometimes within samples. The A2 mating type was found in all provinces where late blight was present. The A1 mating type was found only in Manitoba (1 isolate) and in BC (38 isolates). Unlike the old A1 isolates, those found in BC in 1997 were highly resistant to metalaxyl. These A1 isolates are even more resistant than A2 isolates. Based on assays with 10 ug.ml^?1 of metalaxyl, 14% of all tested isolates were MS, 69.8% were MI and 16.2% were MR. In addition, all provinces contained MS, MI, and MR isolates. No specific mating type or metalaxyl sensitivity level were specific to tomato or potato.     

Relationships between pathotypes and RAPDs, Gpi-allozyme patterns, mating types, and resistance to metalaxyl ofPhytophthora infestans in Canada in 1997

Relationships between pathotypes and other markers such as mating types, metalaxyl resistance, Gpi-allozyme and random amplified polymorphic DNA (RAPD) patterns, were investigated among 33 isolates ofPhytophthora infestans obtained from both potato and tomato across Canada in 1997. Pathogenicity testings, based on virulence against potato differentials carrying single-resistance genes and the size of lesions on leaves, revealed higher pathogenic diversity in 1997 than in the few past years, with more pathotypes and more race-complex isolates. A higher pathogenic diversity was observed within A2 (US-8) than within A1 (US-11) isolates. There was no correlation between the level of resistance to metalaxyl of an isolate and its pathogenicity. Nevertheless, when grouped into metalaxyl-sensitive (MS), -intermediate (MI), and -resistant (MR) classes, both average pathotype complexity and average lesion size were lowest for MS group. RAPD data revealed a good distinction between A1 (US-11) and A2 (US-8) mating type groups, and between isolates from British Columbia and those from other provinces. The detection in the last few years of both A1 and A2 mating types in Canada and the occurrence in 1997 of some isolates sharing different characteristics of both of these two groups suggest that sexual reproduction may have occurred locally.     

Occurrence of the A2 mating type ofPhytophthora infestans in potato fields in the United States and Canada

Ten isolates ofP. infestans collected from blighted potato fields in the U.S. and Canada during the years 1983–1989 were examined for mating type on lima bean, oatmeal, and rye grain media. Two of the ten isolates produced oospores in less than 15 days when cultured in the presence of known A¹ mating types from the USA, Mexico, and Europe, indicating they are A² mating types. When the two A² isolates (one from Pennsylvania and one from Vancouver, B.C.) were cultured singly or with known A² isolates, no oospores formed after 6 weeks of culture. Pathogenicity tests showed that both isolates produced typical late blight symptoms on potato foliage and stems, and there appeared to be no difference in virulence between these two A² and the eight A¹ isolates. Inoculations with a mixture of A¹ + A² sporangia also produced oospores in host tissues. This is the first report of the presence of the A² mating type ofP. infestans in the U.S. and Canada.     

Factors affecting the survival of metalaxyl-resistant strains ofPhytophthora infestans (Mont.) de Bary in Ireland

Summary Five metalaxyl-resistant and five sensitive isolates ofPhytophthora infestans were compared for sporangium production per unit leaf area, sporangium germination and virulence to foliage and to tubers. Differences between the two groups of isolates were significant for sporangium production per unit leaf area only. The metalaxyl-resistant strains produced fewer sporangia per unit leaf area than the metalaxyl-sensitive strains. The implications under field conditions are discussed.     

Susceptibility of potato (Solanum tuberosum L.) foliage and tubers to the US8 genotype ofPhytophthora infestans

Late blight is an important disease of potato(Solanum tuberosum L.) worldwide, and therefore, many potato-breeding programs have prioritized the development of late blight-resistant potato cultivars. Although the emphasis has been to enhance foliar resistance, it is also necessary to evaluate tuber late blight resistance in new breeding lines and new sources of late blight resistance. We report here on the assessment of foliar and tuber resistance and the correlation between these aspects of resistance in a sample of Michigan State University potato breeding lines. Two MSU breeding lines had significantly less infected foliage than the susceptible check cultivars. Tuber susceptibility was significantly different (P < 0.05) only between the most susceptible and the least susceptible breeding lines/cultivars. Foliar and tuber susceptibility to potato late blight were not correlated as low tuber susceptibility was associated both with extremely low (e.g., MSG274-3) and high (e.g., MSE202-3Rus) foliar susceptibility.     

Changes in race structure of Canadian populations ofPhytophthora infestans based on specific virulence to selected potato clones

Summary A collection of 80 isolates ofPhytophthora infestans, representing multilocus genotypes of the pathogen found in Canada from 1994 to 1996, was screened on a series of nine differential potato host cultivars to determine the physiological race of the isolates. A total of 28 pathotypes were found among the 11 genotypes present in the collection. There was a significant increase in the complexity of pathotypes from 1994 to 1996 which reflected the displacement of the US-1 genotype (mean of 2.2 host differentials infected) by the US-8 genotype (mean of 8.1 host differentials infected). Eighteen of the US-8 isolates could overcome all nine of the resistance genes tested. Virulence genes which overcame host resistance conferred by R8 were the rarest in the populations studied. The predominance of complex populations ofP. infestans in Canada implies that the introduction of cultivars with specific R genes for resistance would be an ineffective control measure. AAFC Contribution No. 901     

Evaluation of wildSolanum species for resistance to the US-8 genotype ofPhytophthora infestans utilizing a fine-screening technique

Greenhouse assays were carried out to characterizeSolanum accessions previously reported to be late blight resistant and to identify individuals within the accessions with high levels of resistance to late blight. Foliage of wildSolanum species accessions or hybrid cultivated potato x wild species (diploid or tetraploid) from Mexico, Russia, or South America was inoculated withPhytophthora infestans (US-8) and was scored for severity of foliar symptoms at 7 and 14 days after inoculation (DAI). Mexican accessions (S. brachycarpum,S. pinnatisectum,S. guerreroense, andS.fendleri) were significantly more resistant (1.1% infection at 14 DAI) than either Russian (59.1% infection at 14 DAI) or South American (53.4% infection at 14 DAI) accessions. Moreover, the genotypes within the Mexican accessions tended to be more uniformly resistant whereas genotypes within the Russian and South American accessions tended to segregate for resistance. The more resistant genotypes of Russian and South American accessions (176/546) were retested, and 56 genotypes were identified as having potential for use in potato breeding programs for resistance to late blight. Fifty percent of the selected genotypes were fromS. microdontum accessions PI498124, PI595510, and PI595511. The potential of these species for incorporation into breeding programs for late blight resistance is discussed.     

Response of tubers of five potato cultivars to co-inoculation with US-1 and US-8 genotypes ofPhytophthora infestans

Summary The severity of late blight tuber rot in five potato cultivars (Green Mountain, Sebago, Dorita, AC Brador and Island Sunshine) resulting from inoculation with US-1/US-8 combinations of Canadian isolates ofPhytophthora infestans was evaluated. In addition, the potential for mixed (A1∶A2) inoculations to result in oospore formation was examined. In studies conducted in 1996 and 1997, cv. Dorita (and Island Sunshine in 1997) developed the least amount of surface or internal necrosis relative to the other cultivars following co-inoculation with US-1 and US-8 isolates ofP. infestans. Differences also were revealed among isolate combinations used for inoculation. The combination of a US-1 isolate from Prince Edward Island with a US-8 isolate from New Brunswick consistently produced the least amount of necrosis in tubers of the various cultivars. There was no evidence of oospore production in any of the tubers examined. AAFC Contribution No. 919