Characterisation of Phytophthora infestans Isolates Collected from Potato and Tomato Crops in Tunisia During 2006–2008

Severe late blight epidemics in Tunisia in recent years prompted population studies on the pathogen responsible for this disease, Phythophthora infestans. Characterisation of 165 Tunisian P. infestans isolates collected from 2006 to 2008 was performed for the mating type and mt haplotype, while subsets were analysed for metalaxyl sensitivity (n?=?65), virulence on differential set of 11 R genes of Solanum demissum (n?=?31), aggressiveness on cv. Bintje (n?=?36) and measurement of the radial growth on agar medium at three temperatures (n?=?38). Most isolates from potato and all isolates from tomato had the A1 mating type. The A2 mating type was detected in the north-east and northern areas, but not in the north-west. All the A2 mating type isolates were metalaxyl resistant and seem to be part of a new generation of the P. infestans isolates which are more aggressive, with more complex races, and tolerant to higher temperatures. The increased severity of epidemics during 2006 to 2008 can be attributed to favourable weather conditions during growing seasons, adaptation of new genotypes, widespread phenylamide resistance in potato production regions and most probably incorrect spray programmes. In contrast to the presence of complex pathotypes in two major potato crop regions (north-east and northern areas), the P. infestans population detected in the other regions and in tomato crops was still relatively simple. Compared with the situation in Europe and the American continent, or even compared with neighbouring countries such as Algeria, the genetic changes in Tunisia are still comforting and require strict management decision on late blight control to avoid the spread of new P. infestans populations from Europe or neighbouring countries.     

Characterization ofPhytophthora infestans from Maine during 1999 and 2000

A total of 77 isolates ofPhytophthora infestans was collected in 1999 and 93 isolates were collected in 2000 from growers’ fields, home gardens, and research plots in Maine. Genotype diversity based onglucose-6-phosphate isomerase (Gpi) allozyme analysis was greater in the collections from 2000, with six genotypes identified vs two genotypes identified in 1999. The US-8 (Gpi 100/111/122) genotype constituted 88% of the isolates collected in 1999 and 75% of those collected in 2000. The US-14 (Gpi 100/122) genotype constituted 12% and 7% of the isolates collected in 1999 and 2000, respectively. Three newGpi genotypes constituted 7% of the isolates collected from potato in northern Maine in 2000. A new genotype (A1 mating type,Gpi 100/100) that was aggressive on both tomato and potato was identified from home gardens in western Maine and constituted 11% of the isolates collected in 2000. A1, A2, and A1A2 mating types as well as self-fertile isolates were collected in both years. Self-fertility in artificial culture appeared to be relatively common in field isolatesof P. infestans. Potential origins of the new genotypes are discussed.     

Phenotypes ofPhytophthora infestans in Nepal: mating types and metalaxyl sensitivity

Summary Potato late blight has appeared in epidemic proportions in Nepal since the mid 1990s and fungicides have been reported to be decreasingly effective in managing the disease.Phytophthora infestans isolates were collected from potato crops during 1999–2000 and analysis of 371 isolates for mating types and 270 isolates for metalaxyl sensitivity revealed the presence of both new and old populations. This is the first report on the presence of metalaxyl-resistant isolates in Nepal. The frequencies of A1 and A2 were 83 and 17%, respectively. Metalaxyl-resistant, intermediate and sensitive isolates were recorded as 10%, 12% and 78% respectively. Metalaxyl resistance was distributed in both mating types. Sites with a history of metalaxyl use had a significantly higher number of resistant and intermediate isolates ({ie337-1}, P<0.01) than sites where metalaxyl had not been used. This study confirms the changes in population structure ofP. infestans in Nepal.     

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.     

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.     

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.     

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.     

Races ofPhytophthora in festans in France, 1991–1993

Summary Nineteen races were found among 116 isolates ofPhytophthora infestans recovered from French potato and tomato crops during 1991–1993. The most common races were 1.3.4.7.11. 1.3.4.7.8.11 and 1.3.4.7.10.11 in 1991, race 1.3.4.7.11 in 1992, and race 1.3.4.7.10.11 in 1993. None of the isolates was virulent to either R2 or R6, and only one to R5. The mean number of virulences per isolate was 5.78 in 1991, 4.35 in 1992 and 4.53 in 1993. TheP. infestans population from Brittany showed mainly quantitative changes in the frequencies of major races over the period investigated, but races identified were almost the same in all 3 years. No geographical substructure was apparent, since variation between locations accounted for only 13–22% of the total racial diversity within the Brittany population. This is interpretated as evidence for limited gene flow in this population, which is believed to have evolved from a few founding genotypes.     

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.     

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.     

Mating Type, Virulence, Aggressiveness and Metalaxyl Resistance of Isolates of Phytophthora Infestans in Poland

A total of 93 single-lesion isolates of Phytophthora infestans were collected from late blight diseased potatoes grown in several locations in Poland, in the years 2002–2004. They were tested for mating type, virulence, aggressiveness and metalaxyl resistance. Sixty one percent of the isolates was of the A1 and 39% of the A2 mating type. Regarding virulence, complex races predominated. Most of the isolates were highly and moderately aggressive. Among isolates, tested together with 38 previously collected (1995–2001) ones, 14.5% was metalaxyl resistant, 3.5% intermediately resistant and 82.0% of the isolates was sensitive to metalaxyl. In addition, 14 isolates tested with three simple sequence repeat markers showed high genetic diversity. These data indicate that Polish population of P. infestans is diverse and aggressive, with potential for sexual recombination and spreading of fungicide insensitivity.     

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.     

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.     

Race diversity and virulence complexity ofPhytophthora infestans in Nepal

Summary A total of 251Phytophthora infestans isolates collected during 1999–2000 potato growing seasons from nine potato growing areas of Nepal were tested for virulence using a set of eleven potato differentials. The virulence test revealed the presence of 30 races with the most frequent being 4.11, 4 and 11. Twenty-nine percent of isolates belonged to simple races having none to one virulence factor, whereas 71% had complex virulence factors governed by 2 to 7 genes. Gleason and Shannon indices of 5.24 and 2.59, respectively, showed a moderate level of diversity in NepaleseP. infestans populations. Shannon indices comparison among regional populations revealed significant differences (t-test, P<0.001) in the diversity. The population from hills were the most diverse followed byterai and the high hill populations. The virulence complexities were minimal in Nepalese populations. The implications of these findings in association with the potato cultivation system in Nepal are discussed.     

Characterization and Distribution of Three New Clonal Lineages of Phytophthora infestans Causing Late Blight in Wisconsin from 2009 to 2012

Phytophthora infestans causes late blight of potato and tomato, a disease that has been estimated to cost U.S. potato growers $287.8 million annually. We collected isolates of P. infestans from Wisconsin from 2009 to 2012 and determined distribution of clonal lineages and mating types and sensitivity to the systemic fungicide mefenoxam. We also sought to evaluate the current utility of an analysis of the Glucose-6-phosphate isomerase (Gpi) allozyme locus for predicting mefenoxam sensitivity with the aim of delivering timely information to growers. Overall, 143 isolates were collected from 52 locations in 20 Wisconsin counties from 2009 to 2012. Three clonal lineages, US-22, US-23, and US-24, were identified and were novel to Wisconsin and the U.S. US-22 is of the A2 mating type and sensitive to mefenoxam, with Gpi 100/122. US-23 and US-24 are of the A1 mating type and primarily intermediately sensitive to mefenoxam, with Gpi 100/100 and 100/100/111, respectively. Because of this close correlation and the unique Gpi patterns for each lineage present, we were able to predict mefenoxam sensitivity directly from samples using the allozyme assay and quickly deliver management information to growers. Both mating types were present in Wisconsin in 2009 and 2010 but were spatially separated and no evidence of sexual recombination or soil persistence was detected. The presence of new clonal lineages of P. infestans in Wisconsin indicates a need for continued close monitoring of late blight to facilitate generation of timely information for enhanced short-term and long-term late blight management.     

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.     

Reaction toPhytophthora infestans of potato genotypes with race-specific resistance

Summary Phytophthora infestans isolates collected in Poland in 1987–1995 were evaluated on detached leaflets of a differential series consisting of potato genotypes possessing resistance genes R1–R11. On the same differentials was evaluated over three years a highly virulentP. infestans isolate MP 245. Isolate MP 245 and those from natural populations ofP. infestans were seldom or inconsistently pathogenic to R5, R8 and R9. When leaflets of the differentials were infected with MP 245 the lesions were often small and sporulation weak. A statistically significant interaction was found between years and pathogenicity of MP 245 to individual differentials. Attempts to train the isolate MP 245 for pathogenicity to R9 were unsuccessful. The variation in specific resistance of potato genotypes may complicate the evaluation of their resistance toP. infestans, as well as the evaluation of the virulence spectrum of natural fungus populations. This variation may be due to changes in specific pathogenicity of the fungus or in specific resistance of the potato. Some differentials have a possibly useful resistance toP. infestans.     

Impact of Agronomic Strategies (Seed Tuber Pre-sprouting,Cultivar Choice) to Control Late Blight (<Emphasis Type="Italic">Phytophthora infestans</Emphasis>) on Tuber Growth and Yield in Organic Potato (<Emphasis Type="Italic">Solanum tuberosum</Emphasis> L.) Crops

In order to optimize potato crop management in organic farming systems, knowledge of crop growth processes in relation to limitations and reductions by nitrogen (N) and disease is necessary. This paper examines the effect of different prevention measures (seed tuber pre-sprouting, choice of cultivars: resistance to Phytophthora infestans; earliness of tuber initiation) against disease-related constraints on yields, depending on the N-mediated growing conditions of organic potato crops. Under conditions of a relatively high N supply, accelerating the early development and tuber initiation of potato crops by seed tuber pre-sprouting (yield increase of c. 18–23%) or the selective choice of cultivars with an earlier tuber initiation (yield increase of c. 0–21%) are the most effective strategies in combating late blight. They bring forward the crop development c. 7–10 days, escaping the negative impact of the fungus on tuber growth. Under conditions of a relatively low N supply, these strategies do not affect final tuber yield. The reason for the differences of the effect depending on N supply of the crops is, that the lower the N supply, the shorter the period of time over which tuber growth takes place, independent of whether P. infestans becomes severe or not. Tuber growth of organic crops low in N has mostly ceased by the time late blight becomes potentially important for limiting yield, with the consequence that preventative measures against P. infestans are meaningless. Surprisingly, the level of cultivar haulm resistance to late blight did not affect tuber yields in years with an early and strong late blight development. Probably, the positive effect of a longer resistance to the fungus (c. 1 week) was counterbalanced by a later tuber initiation (also c. 1 week). In years with a moderate late blight incidence, less susceptible cultivars were able to produce higher tuber yields on fields with a lower N availability (c. 17–20%), probably due to higher N use efficiency and a better match of N mineralization and N uptake. Under conditions of a relatively high N supply, the tested cultivars with a higher haulm resistance to late blight were not able to improve tuber yields.     

A detached-leaf method to evaluate late blight resistance in potato and tomato

We developed a detached leaf method for evaluating potato (Solanum tuberosum L.) and tomato (Lycopersicon esculentum Mill.) germplasm for reactions toPhytophthora infestans, the causal organism of late blight. Primary leaves from the third to the sixth node of potato plants, and the fourth to sixth node of tomato plants were excised at the stem. Their petioles were inserted into 14 mm × 100 mm floral aqua tubes containing 9 ml of sterilized distilled water. The leaves in the aqua tubes were placed with abaxial sides down on galvanized metal hardware cloth (12.5 × 12.5 mm mesh). A 12.5 mm sensi-disc containing 50μl of 2 × 10⁴ zoospores was placed in the centers of the terminal leaflet and the second leaflet pair of the potato leaf. A single disc was placed on the center of the tomato leaf. The supporting hardware cloth was placed 2.5 cm above distilled water (2.5 cm deep) in 31 cm × 17 cm × 8 cm clear plastic boxes with tight fitting lids. Leaves on intact tomato and potato plants were inoculated in a similar manner and placed in a mist chamber. Lesion growth was determined 4, 5, and 6 days following inoculation. There were no significant differences in reactions to isolates ofP. infestans on detached and intact leaves of potato cultivars Green Mountain and Kennebec and the tomato cultivar Bonnie Best.     

Impact and Interaction of Nitrogen and Phytophthora infestans as Yield-limiting and Yield-reducing Factors in Organic Potato (Solanum tuberosum L.) Crops

For organic potato producers, the two main challenges are disease and nutrient management. Both are limited by regulations that on the one hand prohibit the use of chemical fertilizers, especially nitrogen, and on the other hand prohibit most synthetic pesticides. Late blight caused by Phytophthora infestans is commonly thought to be the most yield-reducing factor. However, because there is no really effective fungicide available to control late blight, there are virtually no yield loss data available for organic farming conditions. In this paper, the state of the art of organic potato tuber growth under on-farm conditions with respect to disease and nutrient management is summarized by field trials and on-farm surveys on commercial organic crops carried out in the years 1995–1998. Soil nitrogen (N) levels, plant N uptake, disease development of P. infestans and potato yield were measured. Results indicated that N availability was most important in limiting yields in organic potato crops. From on-farm data, a model including disease development, growth duration of the crops until foliage decay and different parameters related to N status of the crop could explain 73% of the observed variation in yield. Only 25% of this variation in yield could be attributed to the influence of late blight. Differences in N availability explained 48%. In conclusion, several points emerged from the results. In organic farming, yields are mainly limited by nutrient availability in spring and early summer. The effects of late blight on yields may often be overestimated and cannot be deduced from results in conventional farming because of the strong interaction with nutrient status. Depending on N availability, tubers stop growing between mid-July (70–90 kg N ha⁻¹ uptake), the end of July (110–140 kg N ha⁻¹ uptake) and mid-August (140–180 kg N ha⁻¹ uptake) due to N limitations. The higher the N status of a potato crop, the longer the growing period needed to achieve the attainable yield and the higher the probability that late blight stops further tuber growth and becomes the key tuber-yield-limiting factor. In the second part of this paper, the interactive effects of soil N availability and the impact of P. infestans on yield in the presence and absence of fungicides from 1996 to 1998 for mid-early main crops are reported. An empirical schematic model of disease impact depending on N availability was developed.