Relative Area Under Disease Progress Curve Research Articles

Ten-Year Trends in Aerial Stem Rot Management: Evaluating Copper-Based Solutions for Sustainable Potato Cultivation

Aerial stem rot (ASR) is an important disease of potato worldwide and is predominantly caused by the soft rot bacteria Pectobacterium carotovorum. It differs from black leg (caused by P. atrosepticum) as it is found later in the season after row closure. The pathogen enters through wounds in the stem and expands outward leaving a distinct black lesion. Lesions spread down the stem as opposed to black leg lesions which spread up the stem from diseased tubers. Ten years of field trials were conducted using a susceptible variety in a randomized complete block design. Treatments consisted of different formulations of copper fungicides starting at row closure and continuing on a seven-day schedule until vine kill. Disease incidence and severity was rated visually, and results were used to calculate the Relative Area Under the Disease Progress Curve (RAUDPC), showing the rate of disease progress. Results showed that the RAUDPC was consistently higher in the untreated check compared to fungicide-treated plots, suggesting these products significantly reduced both disease incidence and severity. However, while the fungicides reduced disease, this did not always translate into higher yields. Nevertheless, factorial analysis of variance of yield by treatment showed significant differences in total yields with fungicide-treated plots outperforming the untreated check with yields at least 3 tonnes/ha higher. There have been few studies on the efficacy of copper fungicides for control of ASR. The findings of this ten-year study should provide growers with the data necessary to make informed decisions on the management of ASR.

Screening of onion (Allium cepa) genotypes to find out novel resistant source against purple blotch (Alternaria porri)

Purple blotch caused by Alternaria porri (Ellis) Cif., 1930 causes huge losses to bulb crop and seed crop of onion (Allium cepa L.). This is a major disease affecting successful onion production in majority of the onion growing areas. To identify resistant source against purple blotch, 34 onion genotypes with good marketable yield based on field evaluation trial in 2017 and allied species Pusa Soumya were screened under protected conditions during kharif, 2018 at insect proof net house facility of ICAR-Indian Agricultural Research Institute, New Delhi. The pure culture of Alternaria porri, isolated from infected leaf samples of onion crop raised under natural epiphytotic conditions during kharif, 2017. Highest change of disease severity index was observed between 1–2 weeks post inoculation (WPI) while highest disease severity index was recorded at 5 WPI suggesting that preventive measures must be employed as soon as disease symptoms appear in the field. The disease severity index ranged from 0.10–33.33 at 1 WPI, which increased to 9.33–89.33 at 5 WPI. The cultivated allied species Pusa Soumya was identified as novel resistance source with DSI value less than 10.0% at 5 WPI. Four genotypes exhibited moderately susceptible reaction, while 20 genotypes were susceptible and 10 genotypes were highly susceptible. The area under disease progress curve (AUDPC) value ranged from 22.33–240.17. Pusa Soumya had the lowest value and among onion lines Arka Kirtiman was found promising. Three onion genotypes had low relative AUDPC value and can effectively be utilized in resistance breeding programme. The average daily temperature between 20–25°C and relative humidity above 75% was found ideal for disease development.

Evaluation of Olive Varieties Resistance for Sustainable Management of Verticillium Wilt

Verticillium wilt resulting from infection by Verticillium dahliae is one of the most devastating soilborne fungi of the olive tree (Olea europaea L.) worldwide. The pathogen infects a wide variety of plants and can survive in the soil for many years, and chemicals cannot control it. Therefore, sustainable disease management strategies are suggested, with the exploitation of host resistance as the most predominant control measure in practice. In addition, disease risk assessment in commonly used plant genotypes is a prominent issue. In this respect, nine commercially grown Greek olive varieties (‘Amfissis’, ‘Atsiholou’, ‘Chalkidikis’, ‘Koroneiki’, ‘Kothreiki’, ‘Koutsourelia’, ‘Mastoidis’, ‘Megaritiki’, and ‘Tragolia’) and one variety of international interest (‘Picual’) were comparatively evaluated for their resistance to V. dahliae. The roots of young plants were immersed in a concentrated conidial suspension in order to perform an artificial inoculation. We evaluated disease reactions in a 140-day assessment period based on external symptoms (disease severity, disease incidence, and mortality) and calculated the relative areas under disease progress curves (relative AUDPC). The process of qPCR was used to evaluate V. dahliae DNA in vascular tissues and plant growth parameters (height and fresh weight). A cumulative stress response was calculated to consider the overall effect of V. dahliae on olive cultivars. The olive varieties resistance to V. dahliae varied significantly, with ‘Koroneiki’, ‘Tragolia’, and ‘Atsiholou’ being the most resistant. Interestingly, most tested varieties showed a significantly low resistance level, suggesting increased risk for the Greek olive industry due to V. dahliae.

Association mapping to detect Phytophthora infestans resistance in tetraploid potato

AbstractPhytophthora infestans (Mont.) de Baryis found in all areas of potato (Solanum tuberosum L.) production in Argentine. Association mapping uses the existing diversity in a a population to identify loci responsible of phenotypic variation of a character. The studied population was formed by 159 genotypes of the germplasm collection of the Instituto Nacional de Tecnología Agropecuaria (INTA) Estación Experimental Agropecuaria (EEA) Balcarce potato breeding program. These genotypes are part of a larger population that was previously analyzed using single‐nucleotide polymorphism (SNP). Phenotypic assessment was carried out in two locations over 3 yr. Environmental conditions allowed the presence of the disease in all year × location combination. Phenotypic variability was found for relative area under disease progress curve (RAUDPC) and yield parameters as, Total weight and Seed weight. Association mapping was carried out on 159 genotypes. Eight significant associations were found using Bonferroni methodology, with values of −log(p) ranging between 4.61 and 5.42 located at chromosomes 3 (solcap_snp_c2_16679), 4 (solcap_snp_c1_6126 and solcap_snp_c2_38243, solcap_snp_c2_54077), 5 (solcap_snp_c2_42374), 6 (solcap_snp_c2_33228), 7 (solcap_snp_c1_7407), and 10 (solcap_snp_c2_63) and eleven significant associations using permutation tests with values of −log(p) between 4.5 and 6.42 located at chromosomes 1(solcap_snp_c1_5150), 2 (solcap_snp_c1_13923), 4 (solcap_snp_c1_4178, and solcap_snp_c2_50004), 5 (solcap_snp_c2_11829 and solcap_snp_c2_50532), 7 (solcap_snp_c2_35057 and solcap_snp_c1_10011), and 9 (solcap_snp_c2_1915). Some of the SNPs detected are adjacent to genes that encode enzymes that participate in metabolic pathways that are activated in response to interaction with pathogens. Those markers might be included in breeding programs related to improve late blight resistance in potato cultivars.

Wild Olive Genotypes as a Valuable Source of Resistance to Defoliating Verticillium dahliae.

Resistance to the defoliating pathotype of Verticillium dahliae has been evaluated in a pool of 68 wild genotypes of olive belonging to the SILVOLIVE collection. Resistance was evaluated by assessing symptom severity using a 0–4 rating scale, estimating the relative area under the disease progress curve (RAUDPC), determining the percentage of dead plants (PDP), and measuring the evolution of morphological parameters in inoculated plants over time. In addition, the density levels of V. dahliae in the stem of root-inoculated genotypes have been quantified by means of quantitative real-time PCR at 35 and 120 days after inoculation (dai). Fifteen genotypes (22%) were cataloged as resistant to V. dahliae (i.e., disease parameters did not significantly differ from those of the resistant cultivar Frantoio, or were even lower). Resistant genotypes are characterized by presenting fewer symptoms and a lower amount of V. dahliae DNA at 120 dai than at 35 dai, indicating their ability to control the disease and reduce the density of the pathogen. The rest of the evaluated genotypes showed variable levels of susceptibility. Overall analysis of all genotypes showed high correlation between symptomatology and the amount of V. dahliae DNA in the stem of inoculated genotypes at 120 dai, rather than at 35 dai. However, correlation at 120 dai was not observed in the set of resistant genotypes, suggesting that resistance to defoliating V. dahliae in olive is based on the occurrence of different mechanisms such as avoidance or tolerance. These mechanisms are valuable for designing breeding programs and for the identification of target genes and resistant rootstocks to better control Verticillium wilt in the olive grove.

Influence of Bacterial Blight on different phenological stages of cowpea

The cowpea bacterial blight is a common disease, but some of its epidemiological parameters have not been totally elucidated. The objective of this study was to determine the influence of the inoculation of the bacterial blight pathogen (Xanthomonas citri pv. vignicola - Xcv) at different phenological stages of the cowpea, as well the effect on its yield components, and the transmission of the bacterium to the seed. Under greenhouse conditions, plant defoliation was observed on the cowpea cultivar BRS Bragança inoculated with 107 CFU of Xcv/mL at the four phenological stages, - VC (plants with primary leaves at nodes 1 & 2), V3 (3rd trifoliolate leaf unfolded at node 5), R2 (50–100% of open flowers), and R3 (one pod at maximum lenght). There was a difference in the relative area under the disease progress curve (RAUDPC) between the phenological stages and the reduction of some production components, such as dry seed mass. The pathogen was isolated from seeds produced in infected plants in all phenological stage, with the highest values ​​of CFU per gram of seeds in the V3 and R3. Under field conditions, two experiments were conducted using the cv. BRS Vita-7 (resistant) and cv. BRS Bragança (susceptible) inoculated with Xcv at the V3 and R2. The disease was more severe in cv. BRS Bragança, with the RAUDPC being higher in plants inoculated at V3 with a reduction of dry seed mass in inoculated R2 and inoculated R3 when compared to their uninoculated counterparts across two growing seasons. Regardless of the phenological stage in which the plant was inoculated, it was possible to detect Xcv in cv. BRS Bragança and cv. BRS Vita-7 seeds. The highest values of CFU per gram were recorded in samples from cv. BRS Bragança inoculated at V3 stage. Results on the presence cowpea's seeds infected by Xcv show that bacterial blight disease is highly seed transmitted and demonstrate the current need to avoid additional spread of the pathogen to new areas where there are favorable climatic conditions in Brazil or to disease-free countries that can result in considerable losses. Thus, the disease management should be used based on seed certification programmes, realizing sanitary tests of cowpea seed to avoid the spread of Xcv and the maintenance of disease-free seed stocks.

Characterization of Early Blight Resistance in Potato Cultivars.

Early blight, caused by the fungus Alternaria solani, is one of the most economically important foliar diseases of potatoes worldwide. In this study, 217 tetraploid old and modern potato cultivars were evaluated for foliar resistance to early blight in field experiments in Pennsylvania in 2016 and 2017. Relative area under the disease progress curve (RAUDPC) was calculated based on visual assessment of foliar disease during the growing season each year. RAUDPC ranged from 0.0090 to 0.7372 in 2016 and from 0.0215 to 0.7889 in 2017, respectively. Significant differences in resistance to A. solani among cultivars were found (P < 0.0001). A significant interaction was found between cultivar and environment (P < 0.0001). Cluster analysis classified the cultivars into five groups: resistant, moderately resistant, intermediate, moderately susceptible, and susceptible. Broad-sense heritability for early blight resistance was estimated as 0.89 with a 95% confidence interval of 0.86 to 0.92. All cultivars were also evaluated for foliage maturity in separate field trials in 2016 and 2017, and a strong negative correlation between early blight resistance and maturity was found. Maturity-adjusted RAUDPC was calculated by regressing maturity on RAUDPC; predicted values more than two standard deviations greater or less than observed values were used to identify cultivars with greater genetic susceptibility or resistance to early blight, respectively, independent of maturity. Although most resistant and moderately resistant cultivars showed late maturity and most susceptible cultivars showed early maturity, a few exceptions were found.

Genotypic Response of Dry Bean (Phaseolus vulgaris L.) to Natural Field Infection of Ascochyta Blight (Phoma exigua var. diversispora (Bubak) Boerema) under Diverse Environmental Conditions in Rwanda

Ascochyta blight, caused by Phoma exigua var. diversispora (Bubak) Boerema, is a serious constraint in the cultivation of the common bean (Phaseolus vulgaris L.) in Rwanda, particularly in the cool and wet highland production areas. In order to identify resistant genotypes, a germplasm evaluation study was conducted to quantify the impact of the disease on phenotypic and agronomic traits under natural conditions. Field screening trials of 39 bush (Types I, II and III) and 36 climbing (Type IV) genotypes from different accessions within and outside the country were conducted at three sites, namely, Rwerere, Nyamagabe and Musanze Research Stations, for two seasons. The relative area under the disease progress curve (RAUDPC) based on evaluations of the disease severity (percentage leaf area infected), was used to evaluate the genotypes. Thirteen genotypes were identified with some level of ascochyta resistance. The study revealed Rwandan genotypes G 2333 and SMC 18 as new sources of resistance to Ascochyta blight. Additional results showed a negative relationship (r = −0.42 and −0.51 for Seasons A and B, respectively) between ascochyta infection and yield. Further relationships were identified between the plant flower colour and seed size to ascochyta resistance. Some of the identified resistant genotypes can be used to introgress ascochyta resistance into susceptible Rwandan market classes of common bean genotypes.

Detection of rust resistance in selected Zimbabwean and ICARDA bread wheat (Triticum aestivum) germplasm using conventional and molecular techniques

Host resistance is the most effective and economical method to minimise yield losses caused by rusts. The aim of this study was to detect the presence of resistance in 75 wheat genotypes. The presence of the genes Sr2, Sr24, Lr34, Lr37, Lr46 and Lr68 was investigated using simple sequence repeat and sequence tagged site markers. Quantitative aspects of resistance to leaf rust were assessed through infection response, disease severity, coefficient of infection (CI), disease incidence (DI), leaf tip necrosis (Ltn) and area under disease progress curve (AUDPC) under natural epidemics. Highly significant (p ≤ 0.001) differences were observed among the genotypes for CI, DI, AUDPC and relative AUDPC (rAUDPC). Twenty genotypes exhibited high levels of adult-plant resistance, showing CI less than 20% and AUDPC less than 300%, with moderately susceptible to susceptible reactions. The most frequently occurring gene was Lr46 (21%), followed by Lr68 (20%), Lr34 (19%), Lr37 (11%), and Sr24 (0%). Selection for Lr34 and Lr46 based on Ltn alone can sometimes be misleading because of its variable expression in different genetic backgrounds. Cultivars grown in Zimbabwe lacked important rust resistance genes.

Evaluation of Greek grapevine cultivars for resistance to Phaeomoniella chlamydospora

In the present study, four Greek (Agiorgitiko, Asyrtiko, Roditis and Xinomavro) and one international (Soultanina) grapevine cultivars (Vitis vinifera L.) were screened for their resistance to Phaeomoniella chlamydospora. Artificial inoculation was carried out by drilling a hole into the trunk and injecting a concentrated conidial suspension into the vessels. Disease reactions were evaluated in an 87-day assessment period, on the basis of external symptoms (disease incidence, disease severity and mortality) and by calculating the relative areas under disease progress curves (relative AUDPC). The extension of vascular browning as well as the isolation ratio along the inoculated vine trunks were also taken into account as additional parameters for evaluating resistance. The results indicated that the resistance of grapevine cultivars to P. chlamydospora varied significantly. ‘Agiorgitiko’ and ‘Soultanina’ were susceptible, whereas ‘Asyrtiko’ and ‘Xinomavro’ were resistant; ‘Roditis’ showed an intermediate level of resistance. Cultivars’ resistance was mostly distinguished in terms of the extension of vascular browning and pathogen isolation ratio. On the contrary, the disease incidence, final disease severity, mortality and relative AUDPC provided less distinctive efficiency in resistance evaluation. The robust methodology presented here could be useful in rapid evaluation experiments for future screening programs to search and recognize natural resistant sources within grapevine genotypes against P. chlamydospora.

Late Blight and Early Blight Resistance from Solanum hougasii Introgressed Into Solanum tuberosum

Late blight and early blight, caused by Phytophthora infestans and Alternaria solani, respectively, are the two most widely occurring foliar diseases of potato in the U.S.A. Resistance to both diseases is necessary if growers are to reduce fungicide applications. Field resistance to late blight has previously been reported in an accession of Solanum hougasii (2n = 72). The putative aneuploid clone E53.61, derived from (S. hougasii x S. tuberosum) x S. tuberosum was obtained from C.R. Brown and crossed with three S. tuberosum clones. Thirty-five hybrid clones were evaluated for foliar late blight resistance at the Russell E. Larson Agricultural Research Center near State College, PA along with the susceptible check ‘Atlantic’ and for foliar early blight resistance in Presque Isle, ME along with the susceptible check ‘Harley Blackwell’ for 3 years (2012 to 2014). The experimental design was a randomized complete block design with two to three replications each year. The US-23 genotype of P. infestans occurred naturally and/or was used in inoculations in PA and plants were infected naturally with A. solani in ME. Relative area under the disease progress curve (RAUDPC) values were calculated based on visual assessment of foliar disease four to five times late in the season each year and subjected to statistical and stability analyses. There were significant differences among clones and the clone x environment interaction was significant for both diseases. Of the 35 hybrid clones evaluated for late blight, 16 were more resistant, 7 were more susceptible, and 12 were as susceptible as ‘Atlantic’. Of those same hybrid clones evaluated for early blight, 23 were more resistant than ‘Harley Blackwell’; the rest were as susceptible. Late blight resistance or susceptibility was independent of the stability of resistance, however, early blight resistance was associated with greater stability. Fourteen clones were more resistant than the check varieties for both late blight and early blight, suggesting that resistance genes for both late blight and early blight have been combined in this genetic material; three of these clones also had high specific gravity and acceptable chip color out of 10 °C storage.

Field and Greenhouse Evaluations of Cucurbit Rootstocks to Improve Verticillium Resistance for Grafted Watermelon

Grafting watermelon (Citrullus lanatus Thunb.) onto resistant rootstocks is used in many areas of the world to overcome soilborne disease losses including verticillium wilt caused by Verticillium dahliae Kleb. Currently, this disease poses a serious risk to watermelon growers in Washington State. To identify resistant rootstocks, the verticillium wilt reactions (chlorosis, necrosis, and wilting) of 14 nongrafted PI accessions including Benincasa hispida Thunb., Cucurbita moschata Duchesne ex Poir., and Lagenaria siceraria Molina Standl. from the U.S. Department of Agriculture National Plant Germplasm System (USDA NPGS); 11 nongrafted commercially available rootstocks; and, nongrafted ‘Sugar Baby’ watermelon (verticillium wilt–susceptible control) were visually assessed in a field naturally infested with V. dahliae at a rate of 17 colony-forming units (cfu) per gram of soil. Typical symptoms of verticillium wilt were observed on all entries. ‘Sugar Baby’ had the highest relative area under disease progress curve (RAUDPC) value (26.80), which was not significantly different from ‘64-19 RZ’, ‘Marvel’, PI 368638, PI 634982, and PI 642045 (average = 10.16). PI 419060 (1.46) had the lowest RAUDPC value, which was not significantly different from ‘Miniature Bottle Gourd’, PI 326320, PI 419016, PI 536494, PI 636137, ‘Strong Tosa’, ‘Strongtosa’, and ‘TZ 148’ (average = 3.36). The mean RAUDPC value of PI accessions (5.49) did not differ significantly from the mean value of the commercial rootstocks (5.68). Microsclerotia typical of Verticillium spp. were observed in the stems of all but one entry (PI 181913). In a greenhouse study, a subset of 12 entries were inoculated with V. dahliae, and by 22 days after inoculation (DAI), ‘Sugar Baby’ had a significantly higher disease rating than all entries except PI 419060, PI 438548, and ‘Titan’. A strong positive correlation was observed between the field and greenhouse studies. Results indicate that commercial rootstocks as well as PI accessions could be used to successfully manage verticillium wilt in Washington; however, grafting compatibility with watermelon must first be ascertained for the promising PI accessions. Although greenhouse-based verticillium wilt assays can be used to help predict rootstock performance in the field, accurate assessment may require manipulating environmental conditions (e.g., temperature and humidity) to approximate field conditions.

Biological control of Verticillium wilt of olive by Paenibacillus alvei, strain K165

In the present study, the efficiency of the biocontrol agent Paenibacillus alvei (strain K165) to suppress Verticillium wilt of olive tree was evaluated in greenhouse and field experiments. In planta bioassays were conducted under greenhouse conditions and revealed that K165 significantly decreased symptoms on the susceptible cultivar ‘Amfissis’ by 44.5 and 51.6 % of the final disease severity index and relative area under disease progress curve (AUDPC), respectively. Thereafter, the suppressive effect of K165 against Verticillium dahliae was studied for two consecutive years (2007 and 2008) in a newly established olive orchard of the susceptible cv Amfissis and the resistant cv Kalamon, naturally infested with V. dahliae. The evaluation of K165 was carried out by recording symptoms, isolations and qPCR quantification of the pathogen in olive tissues. In both years, ‘Amfissis’ trees treated with K165 showed significantly lower final disease severity and relative AUDPC values compared to the non treated controls, whereas, in 2008 decreased symptom severity was associated with significantly lower V. dahliae DNA levels in plant tissues, indicating the suppressive effect of the biocontrol agent. However, no significant suppression was observed in ‘Kalamon’. Pathogen isolations along with qPCR quantification revealed a seasonal fluctuation of V. dahliae biomass in olive tissues with higher amounts occurring in May, and lower amounts in February, August and November. This is the first report of biological control of Verticillium wilt of olive tree under field conditions, associated with reduced pathogen levels inside the xylem tissues.

Anthracnose on almond in Australia:disease progress and inoculum sources of Colletotrichum acutatum

Anthracnose, caused by Colletotrichum acutatum, is an important disease of almond and has caused significant economic losses in California, Israel and Australia. Anthracnose development was monitored for three growing seasons in an almond orchard in South Australia on two almond cultivars, Price and Nonpareil, with up to 80 % of fruit affected in 2004. Lesions, typical of anthracnose, formed on young developing fruit and symptoms continued to appear until the fruit were ca 20 mm long, after which no further lesions developed. Symptoms were observed on leaves, woody tissue showed signs of dieback, but blossom blight was not observed. Maximum disease incidenceperfor, man and Relative Area Under the Disease Progress Curve (RAUDPC) were significantly larger for Price than Nonpareil for each season, but differences in the apparent rates of infection for both cultivars were insignificant for the three growing seasons. The apparent rates of infection were correlated with rainfall and daily temperature for the three years combined but there was no correlation between maximum disease incidence or RAUDPC and these environmental parameters. Considerably more mummified fruit remained on the trees of cv. Price than Nonpareil each year; however, there was no correlation between the number of mummified fruit in one season and maximum disease incidence, RAUDPC or apparent rate of infection, in the following season. C. acutatum was recovered from mummified fruit, peduncles and bark, from both Price and Nonpareil, every month throughout a year-long sampling period. C. acutatum was also recovered from asymptomatic leaves, fruit, bark, buds and blossom, however, less frequently and at lower rates than from mummified fruit and peduncles. Recovery was consistently greater from Price than from Nonpareil for all tissues.

Comparison of Different Parameters for Evaluation of Partial Resistance to Rice Blast Disease

Aim: To evaluate and identify the most suitable parameter for easy and quick recognisation of rice genotypes possessing partial resistance to rice blast disease. Experimental Design: The tested varieties were grown in one meter long single-row plots surrounded by the blast susceptible spreader rows of Karuna, with a spacing of 10 x 5 cm in a Uniform Blast Nursery pattern. The experiment was conducted in a randomized complete block design with three replications. Place and Duration of Study: The experiments were conducted at the Central Rice Research Institute farm, Cuttack, continuously for nine seasons from 1998 to 2001. Methodology: The disease severity was recorded at every alternate day intervals from disease initiation till end of epidemic. The disease scores were subjected to estimation of 12 parameters for evaluation of resistance. The data on 12 parameters for 42 rice genotypes tested across nine seasons were subjected to principal component analysis, in order to classify and ordinate the response of the genotypes and determine the relative Research Article American Journal of Experimental Agriculture, 4(1): 58-79, 2014 59 importance of parameters. Results: The cluster analysis classified the 42 genotypes into 4 groups of A&B as susceptible and C&D as partial resistant (PR) clusters, during each season of study. The PR genotype groups were characterized by lower estimates of Final disease severity(FDS), Mean disease severity(MDS), Area under disease progress curve(AUDPC), Relative AUDPC(RAUDPC), logistic infection rate(r), Gompertz infection rate(k), genotype score on first(PC-1) and second(PC-2) principal components, logit line intercept(logit-a), Gompit line intercept(gompit-a), and higher estimates of days to reach 50% severity in logistic model(T50r ) and Gompertz model(T50k).Ordination of genotypes onto the PC-1 & PC-2 planes recognized 19 genotypes in group C and 12 genotypes in group D as PR, while rest of the 11 genotypes in cluster-A & B were susceptible. The relative importance of the parameters analyzed by factor analysis revealed that FDS, MDS, AUDPC, RAUDPC, r, k and PC-1 were the top ranking parameters with highly significant correlation among them. Conclusion: One can choose any of the above seven top ranking parameters for easy identification of PR genotypes, depending upon the available resources for computation. Among the 42 rice genotypes, 12 in cluster-D and 19 in cluster-C were identified as possessing partial resistance. The technique of principal component analysis, the ordination and positioning of genotypes on the ordination figure emerged as a valuable tool in identification of rice genotypes possessing PR to blast disease.

A Cut-Stem Inoculation Technique to Evaluate Soybean for Resistance to Macrophomina phaseolina.

Charcoal rot of soybean is caused by the fungal pathogen Macrophomina phaseolina. Effective and reliable techniques to evaluate soybean for resistance to this fungus are needed to work toward a management scheme that would utilize host resistance. Three experiments were conducted to investigate the use of a cut-stem inoculation technique to evaluate soybean genotypes for resistance to M. phaseolina. The first experiment compared aggressiveness of M. phaseolina isolates collected from soybean on different soybean genotypes. Significant (P < 0.05) differences among the isolates and genotypes for relative area under disease progress curve (RAUDPC) were found without a significant isolate-genotype interaction. The second experiment compared 14 soybean genotypes inoculated with M. phaseolina in multiple trials conducted in two environments, the greenhouse and growth chamber. Significant (P < 0.05) differences among environments and highly significant (P < 0.001) differences among soybean genotypes for RAUDPC were found. The environment-genotype interaction was nonsignificant (P > 0.05). Soybean genotypes DT97-4290, DT98-7553, DT98-17554, and DT99-16864 had significantly (P < 0.05) lower RAUDPC than 7 of the 14 genotypes. The third experiment evaluated resistance in selected Phaseolus spp. and soybean genotypes. The range of RAUDPC for Phaseolus spp. was similar to that of soybean. The Phaseolus lunatus 'Bush Baby Lima' had significantly (P < 0.05) lower RAUDPC than P. vulgaris genotypes evaluated. The cut-stem inoculation technique, which has several advantages over field tests, successfully distinguished differences in aggressiveness among M. phaseolina isolates and relative differences among soybean genotypes for resistance to M. phaseolina comparable with results of field tests.

Incidence and pathogenicity of races and isolates of Verticillium dahliae in Crete, southern Greece

Classification of 32 Verticillium dahliae isolates originating from 19 plant species in eight different botanical families to races and determination of host range pathogenicity were carried out. The physiological races of isolates were identified using the two differential tomato cultivars ‘Belladonna’ (susceptible to both races 1 and 2 of V. dahliae) and ‘Ace 55VF’ (resistant to race 1, susceptible to race 2 of V. dahliae). Among these isolates, 14 were race 2 (43.8%), 12 race 1 (37.5%) and six nonpathogenic (18.7%) on tomato. The host range pathogenicity of isolates was determined using four differential hosts (eggplant, turnip, tomato (Ve − ) and sweet pepper). Among isolates, five were pathogenic to both eggplant and turnip (15.6%), 21 to eggplant, turnip and tomato (65.6%), five to eggplant, turnip, tomato and sweet pepper (15.6%) and one was pathogenic to eggplant, turnip and sweet pepper (3.2%). The pathogenicity of isolates on the aforementioned five hosts was investigated on the basis of external symptoms and by calculating the relative areas under disease progress curves (relative AUDPC). Results showed that eggplant was the most susceptible, followed by turnip and tomato cv. Belladonna, while sweet pepper and tomato cv. Ace 55VF were less susceptible to all the isolates used. The pathogenicity of isolates varied from highly to mildly virulent on eggplant and turnip while on Belladonna, Ace 55VF and sweet pepper it varied from highly virulent to nonpathogenic. Belladonna exhibited a similar level of susceptibility to races 1 and 2 of V. dahliae, but was more susceptible than Ace 55VF to race 2. Interestingly, the isolates originating from eggplant were clearly more virulent than those originating from tomato and black nightshade on all solanaceous plants tested.

Evaluation of Advance Wheat Lines for Slow Yellow Rusting (Puccinia striiformis f. sp. Tritici)

Stripe rust (yellow rust), caused by Puccinia striiformis f. sp. tritici is one of the most damaging diseases ofwheat in Pakistan. Lack of durable resistance in local wheat varieties is the main reason for stripe rust epidemicwhich could limit yields. The use of genetically resistant wheat varieties is the most economic way of controllingthe disease. Evaluation of 135 advance wheat lines for slow yellow rusting was conducted during croppingseason 2008-2009 under natural epidemics at field locations of Ayub Agriculture Research Institute, Faisalabadand Cereal Crops Research Institute, Pirsabak (Northwest), Pakistan. Resistance level based on final diseaseseverity (FDS) along with other slow rusting parameters relative area under disease progress curve (rAUDPC)and relative infection rate (RIR) was assessed. A total of 25 wheat lines were identified to potentially havedurable resistance with low frequency of disease severity (10-30%) and lower relative AUDPC values (2-66%).Based on the slowing rusting data, RAPD (random amplified polymorphic DNA) primers were used in order toevaluate genetic diversity among 25 lines. Of 20 OPA (Operon series) primers tested, 4 (20%) primers werepolymorphic that showed amplification differences among 25 genotypes. OPA-06 and OPA-04 revealed thehighest polymorphism (67% and 50%) while OPA-02 and OPA-17 exhibited the lowest polymorphism (33% and25%) respectively. From the amplification profile, a total of five RAPD markers were obtained in this study. Asimilarity matrix data depicted that most of these genotypes are genetically very close (60-100%). The 25advance lines identified from slow rusting evaluation with five RAPD markers may have partial resistance genesand can be used as slow yellow rusting lines with longer field life in Pakistan breeding program.

Dynamics of Soybean Rust Epidemics in Sequential Plantings of Soybean Cultivars in Nigeria.

Soybean rust, caused by Phakopsora pachyrhizi, is an important foliar disease of soybean. Disease severity is dependent on several environmental factors, although the precise nature of most of these factors under field conditions is not known. To help understand the environmental factors that affect disease development, soybean rust epidemics were studied in Nigeria by sequentially planting an early-maturing, highly susceptible cultivar, TGx 1485-1D, and a late-maturing, moderately susceptible cultivar, TGx 1448-2E, at 30- to 45-day intervals from August 2004 to September 2006. Within each planting date, disease onset occurred earlier on TGx 1485-1D than on TGx 1448-2E, and rust onset was at least 20 days earlier on soybean planted between August and October than on soybean planted between November and April. The logistic model provided a better description of the temporal increase in rust severity than the Gompertz model. Based on the logistic model, the highest absolute rates of disease increase were observed on soybean planted in September 2006 and October 2004 for TGx 1485-1D and TGx 1448-2E, respectively. Disease severity as measured by the relative area under disease progress curve (RAUDPC) was significantly (P < 0.05) negatively correlated with evaporation (r = -0.73), solar radiation (r = -0.59), and temperature (r = -0.64) but positively correlated with urediniospore concentration (r = 0.58). Planting date and soybean cultivar significantly (P < 0.05) affected disease severity, with severity being higher on soybean crops planted during the wet season than those planted in the dry season. Within the wet season, planting in May and July resulted in a significantly (P < 0.05) lower RAUDPC than planting between August and October. Yields were significantly (P < 0.001) related to RAUDPC during the wet season, whereby an increase in RAUDPC resulted in a linear decrease in yield. This study suggests that selection of planting date could be a useful cultural practice for reducing soybean rust.

Estimating the Level of Susceptibility to Phytophthora infestans in Potato Genotypes

Resistance and susceptibility are closely related terms but differ in their underlying assumptions and measurement. Standardized methods for determining the level of resistance and susceptibility in potato to Phytophthora infestans, the causal agent of late blight, have traditionally been semiquantitative and are not based on a true interval scale, thus making their use in most mathematical and statistical operations inappropriate. Recently, researchers have attempted to develop interval scales using regression analysis of the direct or transformed area under the disease progress curve (AUDPC). In this article, a similar approach is described based on the relative AUDPC (RAUDPC) of one or two reference cultivars and tested using a data set of field trials involving cultivars with varying levels of susceptibility evaluated in different environments in several countries. The coefficient of variation (CV) among trials of the AUDPC was reduced when the RAUDPC was used and even more so when the RAUDPC was made relative to the RAUDPC of cv. Bintje (RaRAUDPC), which was present in all trials. The RaRAUDPC was used in regression models to estimate scale values for eight potato cultivars in 13 to 15 locations (depending on cultivar). The CVs of scale values measuring variation among sites were similar to those of the RaRAUDPC. Using two cultivars gave a slight improvement in CV, which was statistically significant. The scale developed here has ascending numbers for increasing susceptibility, is simple, and can be constructed as a ratio measure, which permitted the calculation of mean, variance, and CV.