Two Sibling Species of the Botrytis cinerea Complex, transposa and vacuma, Are Found in Sympatry on Numerous Host Plants

ABSTRACT Strains of Botrytis cinerea (the anamorph of Botryotinia fuckeliana) were collected from 21 different plant species around vineyards in the Champagne region (France). Strains were analyzed using three new polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) markers that were found by SWAPP (sequencing with arbitrary primer pairs), in addition to 15 other markers (PCR-RFLP, transposable elements, and resistance to fungicides). The markers revealed a high degree of genetic diversity and were used to investigate population structure. The two sympatric species transposa and vacuma, previously identified on grapes in these vineyards, were also detected on many of the plant species sampled. A new type of strain was also detected, having only the transposable element Boty. We did not detect any differentiation between strains from different organs or locations, but the prevalences of transposa and vacuma were significantly different on the different host plants. Fungicide resistance frequencies were significantly different in transposa and vacuma species. This study confirms that B. cinerea is a complex of sibling species and shows that the sibling species occur sympatrically on many host plants. However, the two species seemed to have different pathogenic behaviors. These findings contradict the traditional view of B. cinerea as a clonal population without specialization.     

北京地区草莓灰霉病菌的转座子及其分布频率

为了解北京地区草莓灰霉病菌的转座子类型及其分布,本研究用转座子Boty和Flipper的特异性引物对北京地区2012-2013年从12个草莓园采集和分离的60株草莓灰霉病菌进行PCR扩增。结果表明,北京地区草莓灰霉病菌群体中共存在3种转座子类型:transposa型、Boty型和Flipper型。其中,以transposa型菌株最多,占供试菌株的63.3%,Boty型菌株占供试菌株的28.3%,Flipper型菌株最少,仅占8.4%,未检测到vacuma型菌株。选取属于不同转座子类型的18株菌株测定其对草莓叶片的致病力,结果显示Boty型菌株所致病斑的平均直径显著大于Flipper型。草莓灰霉病菌转座子类型与致病力的关系有待进一步研究。转座子类型的检测为进一步研究灰葡萄孢的遗传多样性及遗传变异提供了基础。     

Stability and fitness of pyraclostrobin- and boscalid-resistant phenotypes in field isolates of Botrytis cinerea from apple

Phenotype stability, fitness, and competitive ability of pyraclostrobin- and boscalid-resistant isolates of Botrytis cinerea from apple were investigated. Stability of resistance was determined after consecutive transfers on potato dextrose agar (PDA) or being cycled on apple fruit. In vitro fitness components mycelial growth, osmotic sensitivity, conidial germination, and sporulation were evaluated on agar media. Pathogenicity, virulence and sporulation on apple fruit were evaluated at both 20 and 0°C. Competition between fungicide-resistant and -sensitive isolates on apple fruit also was evaluated. Resistance to the two fungicides was retained at levels similar to that of the initial generation after 20 and 10 transfers on PDA and five and three disease cycles on apple fruit at 20 and 0°C, respectively. Great variability in individual fitness components tested was observed among isolates within the same phenotype groups either sensitive or resistant to the fungicides but, when compared as phenotype groups, there were no significant differences in the mean values of these fitness components between resistant and sensitive phenotypes except that the phenotype resistant only to boscalid produced fewer conidia in vitro than sensitive isolates. Resistant isolates were as pathogenic and virulent on apple fruit as sensitive isolates. There was no significant correlation between the values of individual fitness components tested and the level of resistance to pyraclostrobin or boscalid, except that virulence at 20°C positively correlated with the level of resistance to the two fungicides. The final frequency of pyraclostrobin-resistant individuals in the populations was significantly decreased compared with the initial generation and no boscalid-resistant individuals were detected after four disease cycles on apple fruit inoculated with a pair mixture of a dual-sensitive isolate and one isolate each of the three phenotypes resistant to pyraclostrobin, boscalid, or both. The results suggest that resistance of B. cinerea to pyraclostrobin and boscalid was stable in the absence of the fungicides and that resistance to the two fungicides did not significantly impair individual fitness components tested. However, both pyraclostrobin- and boscalid-resistant isolates exhibited competitive disadvantage over the dual-sensitive isolate on apple fruit.     

致病疫霉对缬菌胺抗性突变体生物学特性及抗性风险量化评估

 通过研究致病疫霉对缬菌胺抗性突变体的生物学特性,评估致病疫霉对缬菌胺产生抗性的风险。采用菌丝生长速率法检测田间菌株对缬菌胺的敏感性;通过比较抗性突变体在无药条件下继代培养前后对缬菌胺的敏感性来确定其抗性稳定性;采用离体叶片法测定抗性突变体及其亲本敏感菌株的适合度和竞争力;通过量化7个基本抗性因素的风险值评估致病疫霉对缬菌胺的基本抗性风险。结果表明:在2018年从内蒙、河北、贵州、四川、黑龙江等五省(自治区)采集分离的153个菌株和2019年从河北、内蒙采集分离的40个菌株中低抗菌株分别占31.4%和35.0%,敏感菌株分别占68.6%和65.0%,抗性指数均为0.34,抗性倍数分别为1.5和2.1,未发现中抗和高抗菌株;8个抗性突变体在无药条件下继代培养10代后,少数抗性突变体的抗药性不能稳定遗传;6个抗性突变体的适合度均显著低于其亲本敏感菌株;4个抗性突变体与其亲本菌株的孢子囊不同比率混合物分别在离体叶片上继代培养1、3、7代的抗性频率显著低于初始抗性频率或与初始抗性频率无显著差异;致病疫霉对缬菌胺7个基本抗性因素的风险值之和为9,推测致病疫霉对缬菌胺的基本抗性风险为低至中等。建议加强致病疫霉菌群体对缬菌胺的抗性监测,将缬菌胺与不同作用机理的杀菌剂交替或混合使用,以延缓抗药性产生和发展。     

Stability and fitness of anilinopyrimidine-resistant strains of Botrytis cinerea

The fitness of anilinopyrimidine-resistant isolates of Botrytis cinerea compared with that of sensitive isolates, collected from vegetable crops in Greece during 2005, was investigated. Stability of resistance to anilinopyrimidine fungicides was determined after consecutive transfers of the fungal isolates on fungicide-free potato dextrose agar for 16 culture cycles or on fungicide-untreated cucumber seedlings for eight disease cycles. Results showed that after the consecutive transfers of the isolates either in vitro or in vivo sensitivity to cyprodinil was not changed significantly compared to the initial sensitivity in all the isolates tested, suggesting a stable genetically controlled trait. Fitness parameters measured were mycelial growth, spore production in vitro, osmotic sensitivity, virulence, spore production in vivo, percentage of spore germination, and competitive ability of the resistant isolates in four pairs with sensitive isolates both on artificial nutrient medium or on cucumber seedling plants. The measurements of the fitness components in individual isolates showed high variability within both sensitivity groups in all, except virulence, fitness components tested. As a group, resistant isolates showed significantly lower (P < 0.05) mycelial growth and virulence, while they were more osmotically sensitive than the sensitive isolates. In addition the resistant isolates showed higher (P < 0.05) spore production in vivo but there was no difference (P > 0.05) between the two sensitivity groups in spore production in vitro and in the percentage of spore germination. However, the correlation to test if there is any relationship between the values of each fitness component tested and the level of cyprodinil sensitivity of each isolate was for all, except the spore production in vivo, fitness components not significant (P > 0.05). This absence of significant correlation coefficient values suggests that the development of resistance to anilinopyrimidine fungicides did not affect the fitness of the resistant isolates. Competition of the resistant versus sensitive isolates was isolates-dependent, since in two of the isolate pairs the resistance frequency decreased significantly after five culture or disease cycles, while in the remaining two pairs resistance frequency increased significantly after five disease cycles or remained stable for one pair after five culture cycles on artificial nutrient media.     

Screening of <Emphasis Type="Italic">Botrytis cinerea</Emphasis> isolates from vineyards in Israel for resistance to fungicides

Plots in two vineyards in the Golan Heights, Israel were treated with six botryticides during three growing seasons with 3 applications per season. Applications of fenhexamid, pyrimethanil and cyprodinil + fludioxonil were effective, resulting in 52–65% and 53–63% mean reduction in grey mould incidence and severity, respectively. Carbendazim, fluazinam and iprodione were ineffective or slightly effective. Five hundred and sixteen B. cinerea isolates were collected from infected berries or trapped from the air in the vineyards, and profiles of sensitivity to benomyl, fenhexamid, fluazinam, fludioxonil, iprodione and pyrimethanil were established for each of the isolates based on a mycelial growth test. Seventy-four percent of the isolates were sensitive to the six tested fungicides, and the other 26% of the isolates were classified into 10 phenotypes characterized by resistance to one or more fungicides. Resistant isolates showed fitness parameters similar or reduced in comparison to sensitive isolates. Resistance to benzimidazoles and to dicarboximides was the most frequent (up to 25%) and apparently pre-existed in the populations tested. Increased frequency of benzimidazole resistance, but not dicarboximide resistance, was observed following the 3 years of applications of the fungicides. High level resistance to pyrimethanil was present at a frequency of about 2% in both vineyards in the first 2 years of the sampling survey and reached 10% in the third year at Site 2. A few isolates were resistant to fenhexamid or fludioxonil (0.8 or 0.2%, respectively). No strong resistance to fluazinam was detected, although numerous, less sensitive isolates, presumably possessing multi-drug resistance traits, were recovered at higher frequency from the plots treated with fluazinam than from the untreated plots.     

Studies on metalaxyl-resistant Phytophthora infestans in potato crops in south-west Scotland

In 1981 metalaxyl-resistant Phytophthora infestans was found in 44% of the 63 crops examined in south-west Scotland. In 1982, when fungicides containing acylalanines (metalaxyl or ofurace) were not used in the area, it was detected in 15%. Metalaxyl-resistant strains were found in several crops which had been sprayed only with non-acylalanine fungicides or had received no fungicidal treatment. In neither year was resistance associated with a significant breakdown of disease control.
Metalaxyl-resistant isolates of the blight fungus were obtained from tubers from crops in which resistance was detected on the foliage.
In detached leaf experiments, resistant isolates retained their ability to grow on leaves treated with 100 μg/ml of metalaxyl after 52 weekly transfers on untreated leaves, and produced sporangia on leaves treated with up to 2000 μg/ml of metalaxyl or ofurace. In mixtures with sensitive sporangia the proportions remained stable after nine weekly transfers on untreated leaves. However, at concentrations as low as 50 μg/ml, metalaxyl and ofurace reduced the extent of colonization and the numbers of sporangia produced by resistant isolates by an average of 50%.     

32份木薯种质对疫霉根腐病的抗性评价和农艺性状分析

通过田间自然发病和室内离体接种方法对32份木薯种质进行了棕榈疫霉根腐病抗病性评价,结果表明,供试的32份木薯种质中,高抗种质7份,抗病种质7份,中感种质8份,感病种质7份,高感种质3份。其中高抗种质为‘H360’、‘华南11号’、‘华南8号’、‘H588’、‘桂热3号’、‘H873’和‘H971’;抗病种质为:‘F556’、‘H502’、‘GR911’、‘C-4’、‘F10’、‘南植188’和‘H47’。对其中14份抗棕榈疫霉根腐病木薯种质的农艺性状进行了鉴定和评价,结果表明‘华南8号’、‘GR911’、‘F556’、‘C-4’、‘H360’可以进一步加以利用。     

Strains of Botrytis cinerea resistant to dicarboximide and benzimidazole fungicides in New Zealand vineyards

In a survey of New Zealand vineyards at harvest 1985, isolates of Botrytis cinerea resistant to benzimidazole and to dicarboximide fungicides were common. The mean frequency of resistance in the major vine-growing districts ranged from 8 to 41% for benzimidazoles, and from 51 to 59% for dicarboximides. All benzimidazole-resistant isolates showed high levels of resistance (EC50 greater than 100 mg/l carbendazim based on radial growth response), and all dicarboximide-resistant isolates showed low levels of resistance. Two subgroups of dicarboximide-resistant isolates were recognized, distinguished in the first instance by their osmotic response. Low-level resistant isolates, which formed a dense margin on osmotically amended medium, exhibited an EC50 for mycelial growth on iprodione of c. 3-2 mg/l; ultra-low-level resistant isolates, which formed a fibrillose margin on osmotically amended medium identical to that of sensitive isolates, exhibited an EC50 of c. 1-3 mg/1. In agar culture, radial growth rate, and conidial and sclerotial production of both subgroups were similar to those of sensitive isolates. Virulence (lesion size) and conidial production on grape berries were highest in sensitive isolates, intermediate in ultra-low-level dicarboximide-resistant isolates, and lowest in low-level dicarboximide-resistant isolates. Evidence is presented indicating that ultra-low-level dicarboximide-resistant strains have progressively replaced low-level dicarboximide-resistant strains in the vineyard population. The presence of dicarboximide-resistant strains was linked with a partial loss of fungicide efficacy.     

Diversity and fitness of <Emphasis Type="Italic">Plasmopara viticola</Emphasis> isolates resistant to QoI fungicides

The effectiveness of Quinone outside Inhibitor (QoI) fungicides against grape downy mildew in European vineyards has significantly decreased in the last decade. One nucleotide polymorphism, G143A in the cytochrome b gene of Plasmopara viticola, is involved in resistance to QoIs. Previous genetic examination on the mitochondrial genomes showed four major haplotypes (IR, IS, IIR, IIS) coexisting in European vineyards. A resistant allele (G143A) was present in IR and IIR haplotypes. The purpose of the present study was to estimate the diversity of the different mitochondrial haplotypes and their distribution in QoI-resistant populations before evaluating the potential cost of the resistant mutation G143A in P. viticola population. From 2000 to 2004, the frequencies of resistant isolates ranged from 0% to 23.25% with an average of 4.64 % among the populations examined. To evaluate the fitness of sensitive and resistant isolates, a comparison of different biological parameters including latent period, spore production and infection frequency was performed, enabling a fitness index (F_I) to be determined. Resistant isolates exhibited greater infection frequency than sensitive isolates, whereas no significant difference was found in sporulation ability and latent period between sensitive and resistant isolates. To further investigate competitiveness among isolates, an assay including two resistant isolates in different proportion with a sensitive isolate was conducted on eight asexual growing cycles in the absence of a QoI fungicide. The competitiveness of resistant isolates varied according to their fitness parameters, suggesting that there is no noticeable cost of QoI resistance in controlled conditions in Plasmopara viticola.     

Phenotypic variation in Nordic populations of Phytophthora infestans in 2003

A total of 743 single-lesion isolates of Phytophthora infestans were collected in summer 2003 from Denmark, Finland, Norway and Sweden. Most of the isolates were tested for mating type, and subsets were tested for sensitivity to fungicides and virulence (host specific pathogenicity). Approximately 60% of the isolates were A1 mating type in each country. Both mating types were present in 40% of the fields where more than one isolate was tested, indicating strong potential for sexual reproduction. The proportion of metalaxyl-resistant isolates dropped to under 15% from the 60% observed in the early 1990s in Norway and Finland, possibly due to lower selection pressure because of decreased use of metalaxyl. Propamocarb-HCl sensitivity remained unchanged in the Nordic countries compared to the situation in 1997–2000 in Finland. Four isolates collected from Finland and Sweden were able to sporulate in the presence of this fungicide at a concentration of 1000 mg L⁻¹. In Norway and Finland the frequencies of virulence factors and pathotypes remained nearly unchanged since the 1990s, but the mean number of virulence factors per isolate increased from 5·6 to 6·3. In Denmark and Sweden virulence factors 2 and especially 6 were more common than in Norway and Finland. In addition, in the Swedish population the frequencies of pathotypes were quite even while in other countries pathotype 1,3,4,7,10,11 was most prevalent.     

Fitness of Cercospora Beticola Field Isolates – Resistant And – Sensitive to Demethylation Inhibitor Fungicides

Isolates of Cercospora beticola resistant to fungicides that inhibit sterol demethylation (DMIs) were collected from sugar beet fields in Northern Greece. Fitness of these isolates was compared to that of DMI-sensitive isolates. The parameters measured were competitive ability both under growth chamber and field conditions, mycelial growth, spore germination, germ tube length, incubation period, virulence and spore production. The competitive ability under growth chamber conditions was measured for 4 pairs of one resistant and one sensitive isolate. Results showed that after 4 disease cycles, in 2 out of 4 pairs tested, the resistant isolates competed well with the sensitive isolates, but in the remaining two pairs the frequency of the resistant isolate decreased significantly. The competition experiment in the field was carried out by inoculating field plots with a conidial suspension consisting of a spore mixture from all the resistant and all the sensitive isolates used in this study. Results showed that at the end of the growing period the frequency of the resistant isolates had slightly decreased (P < 0.05). The measurements of fitness components of individual isolates showed that the resistant isolates had significantly lower (P < 0.05) virulence and spore production than the sensitive isolates, while no significant differences (P > 0.05) to the remaining 4 fitness components, were detected. With correlation analysis it was determined whether there is a relationship between values of each fitness component and the level of sensitivity to flutriafol of individual isolates. The correlation coefficients for virulence (r = 0.45) and spore production (r = 0.41) were significantly different from 0 (P > 0.05), indicating that resistance to DMIs affected, to some degree, the fitness of the resistant isolates.     

2012年部分麦区小麦白粉菌群体对三唑酮敏感性及其与毒性的关系

 抗病品种和化学防治是控制植物病害最有效的两种措施,病原菌在长期选择压下可改变其群体结构,以逐步适应品种抗性和杀菌剂压力。本研究采用拌种离体叶段法对 2012年采自我国9个省(市)的129个小麦白粉病菌Blumeria graminis f. sp. tritici单孢堆分离物进行了三唑酮敏感性检测,同时采用31个已知抗病基因品种(系)对其进行苗期毒性测定,并对二者之间的相关性进行分析结果表明:129个供试菌株对三唑酮的EC₅₀为 109.97 mg/L,平均抗药性水平达到52.62倍,变异系数为 107.2。所有供试菌株中,99.22% 的菌株产生了抗药性,其中高抗菌株占58.91%,中抗菌株占37.98%。北京菌株的抗药性水平明显高于其他8个省市。利用Popgen1.32软件对9个省(市)小麦白粉病菌群体的毒性多样性分析结果表明,四川群体毒性基因多样性水平最高,Nei基因多样性H值为0.224 1,浙江群体最低,H值为0.096 8。小麦白粉菌群体对三唑酮的敏感性和毒性的相关性分析表明,EC₅₀或EC₅₀变异系数与毒性多样性之间均不存在显著的相关性,但EC₅₀与毒性基因数目之间的关系符合对数方程。此结果可为杀菌剂和抗性品种的合理利用提供依据。     

江西省稻瘟病菌毒性的群体结构分析

用江苏省的吴江、赣榆、通州、高邮和宜兴等5个代表地区的324个稻瘟病菌株接种13个日本已知抗病基因品种，结果表明，江苏省稻瘟病菌对携带有Pi-Ka,Pi-ta,Pi-ta2和Pi-sh抗病基因品种的可致病比例较高，毒力频率达64．2％－97．2％，对携带有Pi-iPi-z和Pi-b抗病基因的品种毒力频率较低，在0－11．9％之间；江苏省稻瘟病菌毒性的群体结构年度间有一定变化，表现为对某些抗病基因品种毒力频率的明显上升或下降。从江苏省10年主栽品种上分离的稻瘟病菌株对13个已知抗病基因品种的毒力频率测定结果表明，不同类型主栽品种上分离的菌株其毒性结构组成有一定差异,表明江苏省水稻主栽品种的抗性类型存在一定差异。     

Impact of fludioxonil resistance on fitness and cross-resistance profiles of Altrenaria solani laboratory mutants

Sensitivity and inherent resistance risk of Alternaria solani to fludioxonil, cross-resistance profiles and the potential implications of resistance mutations on fitness parameters were investigated. Fludioxonil was highly effective against a wild type A. solani field strain both in vitro (EC₅₀?=?0.05 μg/mL) and in preventive applications on artificially inoculated tomato fruit. Mutants with low [Resistance factor (Rf): 15 based on EC₅₀], medium (Rf: 150–300) and high (Rf: > 1000) levels of phenylpyrrole resistance were isolated from the wild type strain at high frequencies following mutagenesis with UV irradiation and selection on fludioxonil containing medium. Resistant isolates retained their resistance levels even after 9 subcultures on fungicide-free growth medium while they could express their resistant phenotypes in planta. Investigation of cross-resistance relationships showed that fludioxonil resistance mutations also reduce the sensitivity of mutant strains to the aromatic hydrocarbon fungicide quintozene as well as the dicarboximides iprodione and vinclozolin. No cross-resistance was observed between fludioxonil and fungicides with different modes of action such as the sterol biosynthesis inhibitors (DMIs) imazalil and flusilazole and the carboxamide boscalid. All fludioxonil resistant isolates were more sensitive to the anilinopyrimidine pyrimethanil, while only two isolates were less sensitive to the QoI pyraclostrobin compared to the wild-type strain. Study of fitness determining parameters showed that resistance mutation(s) had no adverse effects on mycelial growth, conidial germination and sensitivity to osmotic stress while they had a pleiotropic effect on virulence and conidia production in resistant mutants. Results of the present study indicate that fludioxonil is a highly effective fungicide against A. solani, while the risk of resistance development to this fungicide is considered to be medium making fludioxonil an ideal alternative to high risk fungicides such as boscalid and pyraclostrobin whose performance against early blight has already been compromised by resistance development.

     

An hypothesis to account for the behaviour of dicarboximide-resistant strains of Botrytis cinerea in vineyards

Ten trials were established in the 1986 vintage year on vineyards in three vine-growing districts of New Zealand, to examine changes in the frequency of resistance to dicarboximide fungicides of the Botrytis cinerea population under different fungicide programmes. The vineyards chosen encompassed sites with pre-season resistance frequencies ranging from 0 to 97%. Under a non-dicarboximide programme, using dichlofluanid and chlorothalonil, resistance frequency had decreased at harvest irrespective of the initial frequency. In programmes incorporating dicarboximides (chlozolinate, iprodione, procymidone, vinclozolin), the resistance frequency tended to increase if pre-season resistance frequency was low, and decrease if it was high. The hypothesis is presented that for a given dicarboximide programme, resistance frequency tended to one value, the balance value, where selection for resistant strains due to fungicide application balanced selection against the resistant strains due to their low fitness. This value was estimated for the 1986 vintage year to be 38% for a programme where application of dicarboximides was restricted to the period from the veraison (sugar rise), and 64% where dicarboximides were applied from the pre-bunch closure phase. Actual values observed at harvest 1987, after these same programmes had been applied for two consecutive years, were 39% (S.E. 8-5%) and 68% (S.E. 8-8%), respectively.     

The occurrence, characteristics and distribution in the United Kingdom of resistance to phenylamide fungicides in Bremia lactucae (lettuce downy mildew)

Failure to control Bremia lactucae (lettuce downy mildew) with metalaxyl in an intensive lettuce-producing region of Lancashire at the end of 1983 was shown to be due to the occurrence of a high level of resistance to this fungicide (isolates capable of growth at < 100 μg/ml metalaxyl). During most of 1984, metalaxyl-resistant isolates were obtained from numerous sites but all within a 20-km radius of the initial outbreak. Thereafter, at the end of 1984 and during 1985, metalaxyl-resistant isolates were recovered from most major lettuce-producing regions in the UK with protected crops more affected than field crops. AH metalaxyl-resistant isolates tested were identical in their response to fungicide, sexual compatibility type (B2) and virulence phenotype, probably representing a clone from a single origin. The resistant pathotype was virulent on resistance factors R 1-10 and 12-15 but lacked virulence for R 11 and 16-18. This was also the most common virulence phenotype among sensitive isolates collected at the same time. Cross-resistance to other phenylamide fungicides was demonstrated but isolates resistant and sensitive to phenylamide showed a similar response to the unrelated systemic fungicides propamocarb and fosetyl-Al. An F₁ sexual progeny isolate from a cross between a phenylamide-sensitive and a phenylamide-resistant isolate (presumed heterozygous at the locus or loci regulating response to phenylamide fungicides) exhibited an intermediate response to phenylamide fungicides. No isolates of this type were obtained from the field. At the high concentrations affecting spore germination, phenylamide fungicides exhibited lower activity against a resistant isolate compared with a sensitive isolate. The findings are discussed in relation to future control strategies, the population biology of the fungus and possible directions for lettuce breeding programmes.     

Persistent virulence associations in sexual populations of Puccinia coronata

From 1992 to 1999, 18 sets of aecial or uredinial isolates of Puccinia coronata were collected from two sites in Minnesota for analysis of virulence associations. In addition, one set of aecial isolates was collected in Ontario and one in New York. Also, aecial isolates and uredinial isolates collected from scattered locations in Minnesota in 1994 and again in 1995 were bulked for comparison with populations from discrete sites. Isolates were tested for virulence on 26 single- Pc gene oat lines. Virulences to 14 pairs of differential lines were found to be significantly ( P  < 0·05) associated in linkage disequilibrium in at least six of the 24 site × year populations. The significant virulence associations were found among both uredinial and aecial isolates. Linkage disequilibria normally dissipate with repeated generations of sexual reproduction. Finding the same virulence associations repeatedly over years and locations for sexual populations of P. coronata indicates that certain pairs of virulence genes (or avirulence genes) contribute to increased fitness when they occur together, even in the absence of the corresponding resistance genes in the host. Mean virulence complexity did not differ significantly between a site with no known Pc genes in the host population and a site with low frequencies of Pc genes, suggesting little or no selection pressure against unnecessary virulence per se . Means and standard deviations of virulence complexity were similar for aecial and uredinial isolates within sites, which suggests that selection did not strongly favour either heterozygotes or intermediate virulence complexity during uredinial generations.     

Analysis of Colletotrichum musae populations from Brazil reveals the presence of isolates with high competitive ability and reduced sensitivity to postharvest fungicides

In this study, the sensitivity of 218 isolates of Colletotrichum musae to imazalil and thiabendazole was evaluated, as well the fitness and competitive ability of less sensitive isolates. There was a positive correlation between the sensitivity to the two fungicides, but the isolates were more sensitive to imazalil. The estimated effective concentration of the fungicide able to inhibit mycelial growth by 50% (EC₅₀) was used to select four isolates with the lowest and the highest values for both fungicides, which were considered as sensitive (S) and less sensitive (LS), respectively. The level of sensitivity was maintained after 10 successive transfers on fungicide-free medium. Both fungicides were effective in controlling the disease caused by S isolates of C. musae in detached banana fruit when recommended doses were used. However, only imazalil was able to control the disease caused by LS isolates. For both fungicides, analysis of fitness-related variables (mycelial growth, sporulation, germination, and virulence) showed no difference between the groups of S and LS isolates, but a large variation was observed within the group. The LS isolates to thiabendazole that showed a mutation (F200Y) in the β-tubulin gene did not have fitness penalties. Our results allow a better understanding of the sensitivity and fitness of isolates of C. musae from Brazil, and demonstrate the importance of periodic monitoring to determine the frequency of LS isolates in populations, aiming at more effective management of anthracnose in banana orchards in Brazil.     

Structure and evolution of a population of Erysiphe graminis f.sp. hordei

Samples of single colony isolates of Erysiphe graminis f.sp. hordei were collected in Cambridge on three dates in 1985, and tested for the presence of 12 virulence alleles and resistance to two fungicides, triadimenol and ethirimol. The frequency of the virulence V-(La) fell during 1985, while the frequency of V-h. virulence on cv. Triumph and higher levels of resistance to each fungicide and combined resistance to both fungicides rose. Two phenotypes, both of which possessed virulence on cv. Triumph and three unnecessary virulence alleles and had similar sensitivities to ethirimol, but differed in their level of resistance to triadimenol, accounted for 35.9% of all isolates. The high frequency of these phenotypes accounted for most of the observed gametic phase disequilibria between pathogenicity characters. Most individuals with these two phenotypes may be members of the same clone. It was estimated that 25% of the spore population which initiated the autumn epidemic of E.g. f.sp. hordei originated from ascospores formed by sexual reproduction in the summer. It is proposed that genetic drift followed by hitch-hiking selection, due to intense selection for a clone virulent on a newly-introduced cultivar, is a major factor influencing the frequency of unnecessary virulence alleles.