A root inoculation technique to screen potato seedlings for resistance toPseudomonas solanacearum

A seedling inoculation technique for screening potato progenies for resistance toPseudomonas solanacearum was devised, and the effect of certain treatment variables on efficiency of the technique was determined. Concentration of inoculum and age of seedlings were critical factors; with very young seedlings (16 days old) or low concentration of inoculum (10⁵ cells/ml for isolate K-60) many seedlings escaped infection. With older seedlings (25 days old) and high concentrations of inoculum (10⁹ cells/ml), very few or none of the seedlings survived. Inocula containing mixtures of the most virulent isolates, such as S-207, S-213, and S-123, even at relatively low populations (ca. 10⁵ cells/ml), killed most of the susceptible seedlings. In the standard procedure, true seed was sown in rows 3.2 cm apart in 50 x 35 cm flats and seedlings were grown for 20 days in a growth room at 22 C and a 14-hr photoperiod. For inoculation, 5 liters of a cell suspension of two pathogenic isolates ofP. solanacearum (S-207 and S-213, or S-207 and S-123) were added to each flat, and roots were cut immediately with a knife run between and across the rows of seedlings. Inoculated seedlings were incubated in a growth room at 28 C for 2 weeks, and survivors then were moved to the greenhouse for propagation. Stem inoculation of 20 clones derived from plants that survived the standard procedures showed that 16 of them had clear-cut resistance to the bacterium. Tests with 39 hybrid families from crosses of resistant clones with susceptible Mexican and Colombian cultivars showed a relatively high survival rate in progeny from crosses involving three specific resistant parents.     

Characterization of fungicide resistant isolates of Phaeoacremonium aleophilum infecting grapevines in Spain

Fungicide resistance in Phaeoacremonium aleophilum, one of the most frequent fungal pathogens associated with grapevine trunk diseases, was investigated and found to exist in some isolates of the pathogen against a commercial formulation, Escudo^®. The effect of this compound and its two active substances, carbendazim and flusilazole, was first evaluated on the mycelial growth of P. aleophilum. Escudo^®-resistant isolates were estimated at a frequency of 24% in Spanish vineyards. Then, the two active substances were used individually to test their effect on mycelia growth of twelve single-spore cultures originating from six Escudo^®-resistant isolates. Flusilazole (DMI-triazole) did not inhibit mycelia growth of any single-spore cultures of P. aleophilum. Carbendazim (benzimidazole) used alone allowed the growth of the same single-spore cultures that were also resistant to Escudo^®. AFLP characterization of sensitive and resistant single-spore cultures showed genetic diversity within P. aleophilum isolates but no AFLP markers were associated with resistance. New primers set (L2/R1) were designed to partially amplify the exon 6 of the beta-tubulin gene of P. aleophilum. Two different point mutations resulted in glycine (GGC) or lysine (AAA) replacing the glutamic acid (GAG) at codon 198 of the beta-tubulin gene in some of the resistant single-spore cultures studied. Resistant single-spore cultures of P. aleophilum were shown to have different aggressiveness levels as sensitive single-spore cultures by inoculation of wood segments of Vitis vinifera in the presence and absence of fungicide.     

Soft rot and blackleg reactions in diploid potato hybrids inoculated withErwinia spp.

From 1993 to 1996 three groups of potato genotypes were evaluated for resistance toErwinia spp.: (1) 31 interspecific diploid hybrids (28 resistant and three susceptible), (2) five hexaploid or pentaploid somatic hybrids ofSolanum tuberosum (tbr) ×S. brevidens (brd), and (3) eight cultivars. Two evaluation methods were applied: tuber point inoculation with eitherErwinia carotovora ssp.atroseptica (Eca) orE. chrysanthemi (Ech) to test tuber soft rot resistance and stem base inoculation with Eca to test blackleg resistance. Some resistant diploid hybrids and somatic hybrids oftbr × brd were superior to cultivars for both tuber and stem resistance. Tuber and stem resistance toErwinia spp. in the most resistant diploid hybrids were equal to the highly resistant somatic hybrids oftbr xbrd. Tuber resistance to Eca was highly correlated to tuber resistance to Ech (r=0.815***). In two years of evaluation for stem resistance, three diploid hybrids and a derivative of one of the somatic hybrids (USA M 264) failed to develop symptoms of blackleg following inoculation with Eca, Analyses of variance for tuber and stem resistance indicated significant effects of genotype, year and genotype × year interaction. A positive relationship between tuber and stem resistance toErwinia spp. has been found, however the genetic control of resistance in tuber and stem is partially independent. In the case of Eca the correlation coefficient was r = 0.725***. Therefore it should be possible to obtain resistant genotypes to both blackleg and tuber soft rot. Several resistant diploid hybrids were selected from among those tested, which also have several other characters desirable for potato breeders.     

Virulence response to the Mi.1 gene of Meloidogyne populations from tomato in greenhouses

In a comparison of reproductive traits of 29 field populations of Meloidogyne spp. on resistant and susceptible tomatoes, 48% of the populations were virulent against the Mi.1 gene. Virulent populations produced more (P < 0.05) egg masses/plant, eggs/plant, eggs/g root, and showed higher infection frequencies and multiplication rates than the avirulent populations. The fecundity of females did not differ between the resistant and susceptible genotype, and did not change with the virulence status of the populations. On the resistant genotype, the populations collected from the susceptible tomato showed lower (P < 0.05) reproductive traits than those collected from rootstocks followed by those from resistant tomato. Meloidogyne javanica produced more (P < 0.05) egg masses/plant and eggs/g root than Meloidogyne incognita on the resistant genotype. A time course experiment was conducted to determine if root penetration by M. javanica populations with different Mi.1 virulence status is conditioned by the expression of resistance on tomato. Significant differences between populations were observed as early as of 2 days post-inoculation on the susceptible genotype. On the resistant genotype, low juvenile numbers from populations with acquired virulence penetrated the roots up to 4 dpi, but their numbers significantly increased 7 dpi. Juveniles of the natural virulent population followed similar penetration pattern on the resistant and susceptible genotypes. Peroxidases are involved in the oxidative burst typically associated with the hypersensitive response in incompatible host–pathogen interactions. Isoelectrofocusing isozyme analysis revealed an increase in the peroxidase activity on the resistant genotype after inoculation with the avirulent population and to a lesser degree with the populations with acquired virulence in comparison with the natural virulent population.     

青枯病对不同抗性玉米自交系叶片生理特性的影响

以对青枯病抗性不同的两个玉米自交系为材料,研究青枯病对玉米叶片过氧化物酶(POD)活性、超氧化物歧化酶(SOD)活性、丙二醛(MDA)含量、叶绿素含量、可溶性蛋白含量以及电导率的影响。结果表明,接种青枯病菌后,感病自交系黄早四从14 d开始出现感病症状,抗病自交系K12HF526在30 d后才出现轻微感病症状。两个自交系对照组叶片的POD活性、SOD活性和MDA含量表现趋势一致,抗病自交系高于感病自交系。不同自交系对照组的叶片叶绿素含量一致,处理组中感病自交系明显低于抗病自交系,可能是感染青枯病导致叶绿素含量降低。抗病自交系叶片可溶性蛋白含量在接种前后差异不显著,感病自交系接种21 d后显著降低。通过电导率计算伤害率的结果表明,抗性自交系的伤害率远远低于感病自交系。     

Resistance to potato virus Y inSolanum tuberosum spp.andigena

Tests were made on lines ofSolanum tuberosum spp.andigena to determine 1) whether resistance to potato virus Y exists in the Andigena germplasm, 2) the nature of its inheritance, and 3) the type of resistance. Results of the isolation and identification studies indicated that the pathogen involved was a common strain of potato virus Y. Under field conditions susceptible plants frequently escaped infection. However, field exposures over two seasons resulted in the same ratios as tests in which the same progenies were mechanically inoculated. Mechanical inoculation at the seedling stage proved to be a reliable means of transmission and resulted in accurate screening for resistance. Of 641 tub × tub clones tested, all were susceptible to the virus. Of 366 tub × adg clones tested, 170 were resistant and 196 were susceptible. This fits a 13:15 ratio, assuming random chromatid segregation and a single dominant gene conferring resistance. Plants that were resistant following mechanical transmission were also resistant when inoculated by aphids, indicating the reliability of mechanical transmission as a means of screening for resistance. To determine the type of resistance, top-graft and approach-graft tests were made. Failure to recover the virus from resistant plants inoculated by either grafting method suggests that immunity is the type of resistance involved.     

Screening of three sweet potato (Ipomoea batatas L.) cultivars for resistance to different virulence groups of root-knot nematodes (Meloidogyne spp.) under controlled conditions

The susceptibility of three sweet potato cultivars (Ipomoea batatas L.) C4, TIS 3290 and TIS 9162 was evaluated against 156 isolates of Meloidogyne spp. with the aim to include resistant/tolerant sweet potato cultivars in a crop rotation scheme for the management of root-knot nematodes. The nematode isolates corresponded to races 1, 2 and 3 of Meloidogyne arenaria (n = 7), races 1, 2, 3 and 4 of M. incognita (n = 131) and Meloidogyne javanica (n = 18). Also, the isolates of M. incognita were differentiated in virulence groups: Pepper (n = 35), Pepper-Mi (n = 25), Tomato (n = 41) and Tomato-Mi (n = 30), depending on their ability to parasitize resistant pepper and tomato cultivars. The tested isolates of M. javanica parasitized C4 and TIS 3290, but not TIS 9162, whereas M. arenaria parasitized C4 and TIS 9162, but not TIS 3290, and M. incognita was able to parasitize the three sweet potato cultivars tested. C4 was the most susceptible cultivar to all nematode species tested, especially M. incognita, TIS 3290 was the most resistant and TIS 9162 was in between (7.2, 62.9 and 26.9% of resistant plants, respectively). Susceptibility of the sweet potato cultivars showed slight variations depending on the race or virulence group of M. incognita. The results suggest that sweet potato cultivars TIS 3290 and TIS 9162 may be used as rotation crops in fields where root-knot nematodes are present, their selection depending on the Meloidogyne isolates present. The use of resistant sweet potato cultivars would be preferably combined with other management practices to avoid virulence selection in nematode isolates.     

Fusarium dry-rot resistant potato germplasm

The level ofFusarium dry-rot resistance found in important potato varieties has been improved with agronomically acceptableSolanum tuberosum genotypes. Three breeding selections, A74114-4, A76260-16, and BR6316-7 were highly resistant to the two commonFusarium dry-rot species (F. roseum Lk ex Fr. f. sp.sambucinum [Fs], andF. solani var.coeruleum (Sacc), Booth [Fc]). Resistance was determined separately for each of these species. Resistance to Fs was genetically distinct from resistance to Fc since there was no correlation between Fs and Fc reaction for 180 progeny clones from six crosses representing all combinations of resistance. Resistance to each of theseFusarium species was readily transmitted to progeny. Seventy-five percent of the progeny from resistant X resistant parents were also resistant to Fs, while 95% of the progeny from susceptible X susceptible parents were susceptible. Mixed infections withErwinia atroseptica and eitherFusarium species resulted in a qualitative change from a resistant reaction to a susceptible reaction. Bacterial interaction withFusarium spp. is, therefore, an important consideration in determining storage-rot resistance.     

300份玉米种质对两种灰斑病菌的抗性评价

2013～2016年,采用田间人工接种方法,评价300份玉米种质对两种灰斑病菌玉米尾孢(Cercospora zeina)和玉蜀黍尾孢(Cercospora zeae-maydis)的抗性。结果表明,对玉米尾孢表现高抗、抗和中抗的种质分别为10、100和121份,表现感和高感的种质分别为62份和7份;对玉蜀黍尾孢表现高抗、抗和中抗的种质分别为4、33和78份,表现感和高感的种质分别为137份和48份。76.99%的供试种质对玉米尾孢表现为高抗、抗或中抗,61.67%的供试种质对玉蜀黍尾孢表现为感或高感,玉米尾孢对大多数种质的致病力较玉蜀黍尾孢弱。     

Resistance toPseudomonas solanacearum in potato: Infectivity titrations in relation to multiplication and spread of the pathogen

When several wilt-resistant potato clones were tested against a highly virulent strain from Mexico (No. 276) by standard stem inoculation methods, only one clone ofSolanum phureja was resistant. When different inoculum concentrations were introduced quantitatively for infectivity titrations, however, different levels of resistance were clearly defined among clones previously rated as susceptible. Linear regression was used to estimate the ED₅₀ values (dosage required to wilt 50% of the population) for each of seven clones. These values ranged from 3 and 100 colony-forming units (CFU) for Katahdin and Russet Burbank, respectively, to 2.1 x 10⁶ CFU for S.phureja clone 1386.15. The distribution of bacteria in stems of Russet Burbank plants inoculated with the compatible strains 276 and K-60 and the incompatible strains B1 and S-210 was very different. In all cases, incompatible bacteria were not detected 10 cm above the inoculation point in the stem, even by 12 days after inoculation, whereas compatible strains multiplied rapidly at this site by 6 days. At the inoculation site and at sites immediately adjacent to it, however, incompatible bacteria attained populations that only differed by one order of magnitude from those of compatible bacteria. Similar results were obtained when a resistant clone (MS 118.24) was inoculated with the virulent strain K-60. Thus, resistance was characterized both by reduced acropetal spread and by tolerance to large numbers of the bacterium.     

Evaluation of potato clones for severity of Verticillium wilt, yield and specific gravity in Maine

In 1994 and 1995, the effect of Verticillium wilt, caused byVerticillium dahliae andV. albo-atrum, on tuber yields, number and weight of U.S. No. 1 and B size tubers, and specific gravity was studied in northern Maine, an area with a short growing season. Seven clones (four resistant and three susceptible) were evaluated in a split-plot design with three replications. Clones were the whole-plot factor, and seed pieces in sub-plots were either uninoculated or inoculated with 50 ml of 4 × 10⁴ cfu/mlVerticillium spp. at planting. Individual plants were scored for Verticillium wilt symptoms before harvest on a 1= <3% wilt to 10= >97% wilt. Differences among clones for wilting and specific gravity were significant. The inoculation treatment had no effect on any of the tuber traits measured. However, there were significant clone x inoculation interactions for most tuber traits. Reductions in yield, weight and number of U.S. No. 1 potatoes, and specific gravity were greater in the Verticillium wilt susceptible clones than in the resistant clones. These results suggest that breeding clones with resistance toVerticillium spp. will reduce yield losses, while maintaining tuber size and specific gravity under disease pressure.     

Differential response of Cucumis melo to Fusarium oxysporum f. sp. melonis race 1.2 isolates

Fusarium wilt incited by Fusarium oxysporum f. sp. melonis (Fom) causes severe losses in melon crops. Four physiological races of Fom have been identified: 0, 1, 2 and 1.2. In most cases, resistance to race 1.2 has been described as recessive, polygenic and not race specific. However, some evidence of race-specific effects within melon resistance to race 1.2 has been reported. In this work, we study these effects and assess whether they are due to race-specific resistance. Seeds were obtained from 14 melon accessions that exhibit some level of resistance to race 1.2, and from the lines ‘Charentais-Fom1’ (resistant to races 0 and 2), ‘Charentais-Fom-2’ (resistant to races 0 and 1), and ‘Dinero F₁’ (with partial resistance to Fom race 1.2). Melon seedlings were artificially inoculated using two different procedures: ‘continuous shaking’ and ‘tray immersion’. Symptom severity was assessed on leaves using a rating scale from 0 (no symptoms) to 4 (death of the plant). Symptoms were recorded weekly over the four-week period following the first appearance of symptoms and the area under the disease progress curve (AUDPC) was calculated. Six Fom isolates (3 from pathotype Y and 3 from pathotype W) were used in the inoculation. The less aggressive ‘tray immersion’ procedure seems to be more appropriate for detecting the typically small resistance factors of this type of polygenic partial resistance. ‘Kogane Nashi Makuwa’, ‘BG-5384’, ‘Shiro Uri Okayama’, ‘C-211’ and the control, ‘Dinero F₁’, showed a high level of resistance to all Fom isolates. However, some genotype × isolate effects were also detected. ‘Baza’, when inoculated with isolate Fom 9302, and ‘Korça, when inoculated with Fom 37mls.1.2W, showed resistance levels similar to that of ‘Dinero F₁’; this effect was not observed when ‘Baza’ and ‘Korça’ were inoculated with other isolates. These results are characteristic of race-specific resistance and offer evidence for the presence of this type of resistance to Fom race 1.2 in melons.     

部分国内外小麦种质资源对燕麦孢囊线虫的抗病性

燕麦孢囊线虫是小麦生产上的重要病原线虫。为筛选抗燕麦孢囊线虫的小麦种质资源,采用室内接种方法测试了104个国外小麦品种（系）和37个国内小麦品种对燕麦孢囊线虫的抗病性,并利用不同抗性评价标准在自然病田对37个国内小麦品种的抗病性进行了评价。室内接种试验表明,104份国外小麦品种（系）中有高抗品种（系）14个,中抗品种（系）12个,中感品种（系）27个,其他均属高感,37个国内参试品种均属感病品种。自然病田鉴定结果表明,采用不同的评价标准对37个国内品种的评价结果具有较大差异。依据Andersen等的评价标准,37个小麦品种中有7个品种表现感病,30个品种表现抗病,与室内接种测试结果差异较大;依据Ireholm的标准,仅良星66表现抗病,其他品种均表现感病,与室内测试结果接近;依据Soriano等的标准,37个品种均表现感病,与室内测试结果相同。建议在筛选抗孢囊线虫小麦资源时首先采用Andersen等的标准进行初筛,然后利用Soriano等的标准进行田间评价。     

Assessing fungicide resistance in populations of Alternaria in Idaho potato fields

Early blight, caused by the fungus Alternaria solani and brown leaf spot, caused by Alternaria alternata, are important diseases of potato crops in Idaho. In recent years growers have reported a reduction in efficacy of fungicides traditionally used in the past decade to control early blight. In 2009, a collection of A. solani 39 isolates were screened for resistance to azoxystrobin, pyraclostrobin, boscalid and famoxadone. Fungicide sensitivity testing was done using spiral plate dilution gradients. Results showed that of 39 isolates screened, all were resistant to azoxystrobin and three were resistant to boscalid. None were resistant to pyraclostrobin or famoxadone. In summer 2010, more isolates were collected (9 A. alternata and 26 A. solani) and the survey was expanded to include more fungicides with four different modes of action that targeted succinate dehydrogenase (SDH), methionine biosynthesis, mitochondrial respiration and multi-site contact activity. New isolates of A. solani and A. alternata were also collected from two additional sites. The results showed that 57% of the isolates were resistant to boscalid as well as an average of 63% of the isolates being resistant to the strobilurin fungicides. Seven and 15% of isolates were resistant to penthiopyrad (an SDH inhibitor), and pyrimethanil (a methionine biosynthesis inhibitor), respectively. However, none of the isolates were resistant to fluopyram (an SDH inhibitor) or a mixture of fluopyram and pyrimethanil. Although there appears to be cross resistance developing in Alternaria spp. to some of the new SDH inhibitors like penthiopyrad, others such as fluopyram are still showing limited to no resistance development in Alternaria spp. in Idaho.     

Screening of mungbean germplasm against mungbean yellow mosaic India virus and its vector Bemisia tabaci

Yellow mosaic disease (YMD) caused by whitefly (Bemisia tabaci) transmitted mungbean yellow mosaic India virus (MYMIV) is an important constraint of mungbean in Pakistan. One hundred sixty-two mungbean genotypes from eight different geographic regions were evaluated for resistance to MYMIV and its whitefly vector B. tabaci. Resistance levels were assessed by visual scoring of symptoms in the field under natural conditions, in net-house by artificial transmission through whitefly and in greenhouse using graft inoculation for two consecutive years in 2008 and 2009. None of the tested genotypes was found to be disease free, while considerable variation in responses was observed among the genotypes and testing methods. All genotypes were found to be systemically infected in the field. In 2008, 32 genotypes were found to be resistant and 67 were moderately resistant with severity index (SI) values ranging from 1.0 to 1.4 and from 1.5 to 2.4, respectively. In 2009 34 genotypes were found to be resistant and 38 were moderately resistant with the same SI value ranges as in 2008, and the remaining genotypes were moderately susceptible to highly susceptible. However, only 35 genotypes were found to be moderately resistant with SI values of 1.5-2.4 and latent periods (LP) of 17-21 days when evaluated using artificial transmission through whiteflies. Similar results were obtained when these genotypes were evaluated using graft inoculation. Significant differences in adult whitefly densities among tested genotypes were also observed during both years but no correlation between the number of whiteflies and disease severity was observed. The results indicated that these genotypes did not have a high level of resistance against MYMIV, however, they may provide sources of partial resistance which can be exploited in the breeding programmes to develop mungbean genotypes resistant to MYMD or they can be used directly as varieties to manage MYMD after evaluation for acceptable agronomic characteristics, adaptation and stability in various regions.     

Bacterial ring rot disease development in resistant and susceptible cultivars

In two greenhouse tests, we compared cell populations ofCorynebacterium sepedonicum in inoculated cultivars which had failed to show symptoms in field inoculation tests against cultivars that produce well-defined symptoms. Both population studies showed that cell numbers as high as 1 × 10⁹ colony-forming units per gm of fresh weight (cfu gm^-1) were reached in some cultivars prior to symptom development. In one of these tests, conducted when day length averaged 14 1/2 hrs, some cultivars failed to show symptoms 11 wks after inoculation but were found to have cell populations as high as 3.2 × 10⁸ cfu gm^-1 7 wks after inoculation. In another study, root and seed piece inoculations were compared in susceptible and resistant cultivars and only minor differences in symptom expression due to mode of entry of the pathogen were found.     

玉米抗瘤黑粉病生理生化特性及基因表达分析

采用盆栽试验，以玉米高抗瘤黑粉病自交系齐319和高感自交系掖478为材料，采用注射法接种瘤黑粉菌，从生理生化及防御酶和相关抗病基因转录表达等方面，分析不同抗性玉米种质对瘤黑粉病的抗病特征。结果表明，接种后，2个自交系的苯丙氨酸解氨酶(PAL)、超氧化物歧化酶(SOD)和多酚氧化酶(PPO)活性增强，可溶性糖和脯氨酸(Pro)含量增加，而可溶性糖(SS)含量降低。利用RT-PCR对SOD、PAL、PR-l、PR-2a、COI1和LOX基因的相对表达量分析表明，接种瘤黑粉病菌后，不同抗性材料中抗病相关基因的转录水平均显著提高，抗病材料中的表达时间基本均早于感病材料，基因的转录水平均随着接种时间的增加而上升。在接菌后期，感病材料中防御酶基因和抗病相关基因的表达量大于抗病材料。     

Evaluation of Cluster Bean Genotypes for Resistance to Charcoal Rot Caused by Macrophomina phaseolina Using Different Host Inoculation Methods

Abstract

Ten different isolates of Macrophomina phaseolina from cluster bean (Cyamopsis tetragonoloba) selected on the basis of wide geographical distance and potassium chlorate sensitivity, showed remarkable variation in virulence on the susceptible cluster bean genotype, FS 277. Four different host inoculation techniques were employed for resistance screening of six elite cluster bean genotypes under artificial infection conditions using the most pathogenic isolate, MP-6D. The symptoms induced in the plant tissues from seedling to maturity following artificial inoculation clearly simulated lesions produced under natural conditions. Both greenhouse and field experiments demonstrated significant differences in all parameters-plant weight, plant death, root lesion length and stem lesion length-in all the genotypes tested from seedling stage to pod set. The host inoculation methods described in this study might be useful for screening and analysis of crop varieties' resistance to various other disease infestations. The present investigation involved evaluation of pathogenicity of isolates of M. phaseolina as well as a critical examination of occurrence of resistance to charcoal rot in selected cluster bean genotypes. This is the first report on cluster bean resistance to M. phaseolina.     

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.