US-8 and US-11 genotypes ofPhytophthora infestans from potato and tomato respond differently to commercial fungicides

Isolates ofPhytophthora infestans collected in Canada in 1997 from both potatoes and tomatoes, were tested on potato leaf discs for their response to an equal active ingredient concentration (10 μg a.i./mL) of the following commercial fungicides: Acrobat MZ (Dimethomorph and Mancozeb), Ridomil Gold (metalaxyl-m), Dithane (Mancozeb), Curzate (Cymoxanil), Bravo (Chlorothalonil), and Tattoo C (Propamocarb and Chlorothalonil). Relative percent leaf infection values, estimated on fungicide-treated vs fungicide-free leaf discs, were compared among isolates from the US-8 and US-11 genotypes isolated from the two host plants. Based on an equal concentration of each fungicide’s active ingredients, variations in relative percent leaf infection were recorded between US-8 and US-11 genotypes, and between potato and tomato isolates within each genotype. Bravo and Tattoo C used with similar active ingredients concentrations were the most inhibitory to all groups of isolates. Dithane and Ridomil Gold provided uniform low inhibition againstP. infestans when tested on potato leaf discs. The different behavior ofP. infestans isolates from potato vs tomato suggests that management of late blight in these two important crops must take such differences into consideration. In particular, the nature and concentration of the fungicides to be applied must take into account any information available about genotypes present on each crop.     

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     

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.     

Novel genotypes ofPhytophthora infestans in Canada during 1994 and 1995

Populations ofPhytophthora infestans, causing late blight of potato, changed significantly in Canada in the mid-1990s. The US-1 genotype (A1, metalaxylsensitive [MS]) was displaced by novel genotypes (mainly US-8) in much of the country in 1994 and 1995. Many of the new genotypes were insensitive to metalaxyl and were of the A2 mating type, although metalaxyl-insensitive (MI) Al isolates of the gll (or US-11) genotype were found in British Columbia. A total of 14 unique multilocus genotypes were described over the two years of the study based on mating type (Al or A2), metalaxyl sensitivity (MS or MI), allozyme banding patterns (Gpi andPep), and random amplified polymorphic DNA (RAPD) markers. Six of these genotypes could be distinguished by allozyme banding patterns at theGpi locus alone; RAPD analysis revealed additional variation within allozyme groupings and defined additional genotypes. Among 82 isolates collected from various Canadian provinces in 1994, US-1 and US-8 constituted 30% and 49% of isolates, respectively; other genotypes occurred in minor frequency. Among 50 isolates collected in 1995, US-1 and US-8 constituted 10% and 74%, respectively. Isolates of gll (US-11) were found in British Columbia in 1995. Diversity of genotypes was greater in 1994 (11 genotypes) than in 1995 (six genotypes). We hypothesize that sexual reproduction may have been responsible for generating some of the genetic variability inP. infestans populations, during a time period in which A2 isolates first migrated into many provinces in Canada.     

Impact of seed potatoes infected withPhytophthora infestans (US-1 or US-8 genotypes) on crop growth and disease risk

As the surface area with late blight symptoms of seed tubers increased, the incidence of plant emergence decreased. This relationship was more dramatic with the US-8 genotype than the US-1 genotype ofhytophthora infestans, causal agent of late blight. However, when seed tubers were inoculated with both US-1 and US-8 isolates, the plant growth responses were generally intermediate to those obtained with the two genotypes alone. More resistant cultivars (eg. Dorita, Island Sunshine, and Sebago) had greater plant emergence and less seed rot than more susceptible cultivars (eg. Green Mountain and Shepody) but the response differences between resistant and susceptible cultivars was less clear with US-8 as most cultivars had 100% seed rot. Seed handling treatments affected crop growth and disease risk; as the inoculum levels and in some cases, the pre-planting storage period increased, plant emergence decreased generally due to greater seed rot incidences. Of the one registered seed treatment fungicide and several non-registered and/or experimental fungicides tested, only two significantly improved plant emergence. During three years of study, “diseased” seed treatments generally led to seed rot except in 1996 when one plant in one experimental plot of infected seed had subsequent spread of the disease to the foliage. Furthermore, within one week, a field epidemic developed outwardly from the single infected plant due to the occurrence of favourable weather conditions. Further research opportunities are discussed relative to the impact of infected seed on crop growth and disease risk     

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.     

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.     

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     

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.     

Changes in metalaxyl resistance among glucose phosphate isomerase genotypes ofPhytophthora infestans in Canada during 1997 and 1998

A total of 930 isolates ofPhytophthora infestans collected during 1997 and 1998 from commercial potato and tomato fields, home gardens, and potato storages in Canada was analyzed for glucose phosphate-isomerase (Gpi) genotype, mating type, and resistance to metalaxyl. Both mating types and diversity in resistance to metalaxyl were detected within and amongGpi-genotypes. Pathogen populations were dominated by the 100:111:122Gpi-genotype (characteristic of US-8) in all the provinces where late blight was detected, except in 1997 in British Columbia (BC) where the 100:100:111Gpi-genotype (characteristic of US-11) predominated.Gpi-genotypes 100:111, 111:122, and 100:122 (characteristic of US-7, US-10 and US-14/US-17, respectively) were less common. Nearly all of the isolates with the 100:100:111Gpi-genotype were the A1 mating type, except for a few collected in BC in 1997. A majority of isolates with the 100:111:122Gpi-genotype were A2 except for a few found in Québec in 1998. Based on colony growth on media amended with 1,10, or 100 μg/ml metalaxyl, the 100:100:111Gpi-genotype isolates had higher levels of resistance to the fungicide in 1998 compared to 1997. In 1998, level of metalaxyl resistance was higher within the 100:100:111Gpi-genotype whereas it was lower within the 100:111: 122Gpi-genotype, when compared to 1997. BC was the only Canadian province where an increase in metalaxyl resistance within the 100:111:122Gpi-genotype was recorded between 1997 and 1998. Isolates from leaves had higher metalaxyl resistance levels than stem or tuber isolates, regardless of genotype.     

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.     

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.     

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.     

Variability of diffusive leaf resistance and transpiration rates of various potato genotypes and its possible contribution to water economy

Summary Diffusive leaf resistance and transpiration rate were assessed in various potato genotypes includingS. tuberosum cvs. Atlantic, Cara, Alpha and Désirée,S. phureja clone PP5, a hybrid of Atlantic X PP5, and two anther-derived tetraploids of PP5. Resistance and transpiration were assessed with an LI-1600 steady state porometer on well-watered plants and differences in leaf resistance and transpiration rate among genotypes were obtained. These results imply that water economy may be improved by genotypes exhibiting small water losses. Transpiration rates calculated by dividing the weight of water transpirted per second by the area of the plant's leaflets, were lower than transpiration rates of young expanding leaflets assessed with the porometer. Nevertheless, the relative differences among genotypes were evident and a similar relationship among genotypes was maintained in the two measures. Cv. Alpha had lower transpiration rates, compared to Cara and Désirée, corresponding with its greater stomatal resistance.     

Large scale commercial production of potato minitubers, using in vitro techniques

Summary Large scale, commercial production of potato minitubers, cvs Spunta, Jaerla and Kennebec, has been carried out in VITRO HELLAS S.A. since 1995, according to protocols based on in vitro methods. Apical meristems were cultured on MS medium supplemented with 1 μM IBA, 20 g/l sucrose and 6 g/l agar (pH 5.8). Subculture was carried out every 15 days. Multiplication rate was 4–5 per cycle. Self-rooted microplants were transferred into screen houses (200 plants/m²) in a mixture of peat and perlite 1∶1 (v/v). Cultivation period was 15 August to 15 November each year. During the period 1995–1997, the number of minitubers produced/plant was 2.07 for cv. Spunta, 1.85 for cv. Jaerla and 2.52 for cv. Kennebec and their average weights were 10.8 g, 10.9 g and 9.8 g, respectively. The percentage of minitubers<10 mm in diameter was 2%, 50.1% were 10–20 mm and 47.9% were>20 mm.     

Variation for 2n pollen production and male fertility in wildSolanum germplasm resistant toPhytophthora infestans (Mont.) de Bary (US-8)

Wild potato species possess genetic variability for valuable traits including resistance toPhytophthora infestans, the causal agent for potato late blight disease. Breeding schemes using 2n gametes are a powerful tool for introgressing these traits. Forty-five plant introductions (PIs) across 12Solanum species representing various Endosperm Balance Numbers (EBN) and having resistance toP. infestans (US-8, isolate ND 95-2) were screened for 2n pollen production and male fertility. Species evaluated were 2x(lEBN)S. bulbocastanum, S. cardiophyllum, S. pinnatisectum, andS. trifidum; 2x(2EBN)S. berthaultii, S. megistacrolobum, S. microdontum, andS. verrucosum; 4x(2EBN)S. fendleri andS. stoloniferum; 6x(4EBN)S. guerreroense; and 2x(unknown EBN)S. polyadenium. Acetocarmine staining and cytological analyses were used to determine 2n pollen production and pollen stainability from genotypes grown across three locations. Based on frequency, genotypes were assigned to 2n pollen production categories (0≤1%, Low=1%–10%, Medium=11%–60%, and High≥60%) and pollen stainability categories (0≤1%, 1=196–5%, 2=6%–10%, 3=11%–20%, 4=21%–50%, and 5≥50%). Eleven of the 12 species and 37 of the 45 PIs examined contained genotypes producing ≥1% 2n pollen with ranges of 0%–63.6% and 0%–81.8%, respectively. Dependence was found between location and pollen stainability, EBN and pollen stainability, but not for location and frequency of genotypes with 2n pollen. Stability for 2n pollen production was observed across environments. Among the 1EBN germplasm, dependence was found between 2n pollen and pollen stainability. Resistance toP. infestans was found to be independent of 2n pollen production; therefore, identifying genotypes combining 2n pollen production, fertility, and resistance toP. infestans was possible.