Nadav Nitzan

Effect of Seed-Tuber Generation, Soilborne Inoculum, and Azoxystrobin Application on Development of Potato Black Dot Caused by Colletotrichum coccodes

The effect of azoxystrobin on potato black dot and the role of seed- and soilborne inocula of Colletotrichum coccodes in the development of black dot were evaluated in the field using two potato seed generations (generation 1 and 3) of the susceptible cvs. Norkotah Russet and Russet Burbank over 3 years (2002 to 2004). Plants of Norkotah Russet and Russet Burbank treated with azoxystrobin had 13 and 23% higher yields, respectively, than nontreated plants in 2003. Disease severity on both cultivars was reduced 19 to 81%, and 22 to 81% on above- and belowground stem sections, respectively, when plants were treated with azoxystrobin. Plants of both cultivars that were treated with azoxystrobin had 9 to 26% less infected progeny tubers than the nontreated plants. These results indicated the efficacy of azoxystrobin to reduce black dot severity on both stems and progeny tubers. The roles of seed- and soilborne inocula in disease development were evaluated in 2003 and 2004 using generation 1 and 3 seed tubers. The incidence of C. coccodes in generation 1 mother tubers of Norkotah Russet and Russet Burbank were 2 and 16% in 2003, respectively, and 0 and 30% in 2004, respectively. The incidence of C. coccodes in generation 3 mother tubers of Norkotah Russet and Russet Burbank were 14 and 49% in 2003, respectively, and 12 and 38% in 2004, respectively. Generation 1 plants of Norkotah Russet had 36 and 13% greater yield than generation 3 plants in 2003 and 2004, respectively. In 2004, generation 1 plants of Norkotah Russet and Russet Burbank had 26 and 15% greater disease severity, respectively, on belowground stem than generation 3 plants. Generation 1 plants of Norkotah Russet had 7.5 and 11% more infected progeny tubers in 2003 and 2004, respectively, than generation 3. Significant differences for yield reduction and incidence of infected progeny tubers between the two seed generations were not recorded for Russet Burbank, suggesting that the effect of inoculum source of C. coccodes on black dot severity may be cultivar specific.

Resistance to Root Galling Caused by the Powdery Scab Pathogen Spongospora subterranea in Potato

Potato (Solanum tuberosum) selections (clones and commercial cultivars) were examined for resistance to root galling, caused by the powdery scab pathogen Spongospora subterranea f. sp. subterranea in seven field trials conducted between 2003 and 2007 in the states of Washington and Idaho. Four industry reference cultivars-Shepody, Russet Burbank, Russet Ranger, and Umatilla Russet-were used as susceptible standards. Every year, selections less susceptible than the standards were considered resistant and progressed to the next season. Selections that did not demonstrate resistance in at least two consecutive trials were discarded. Eight potato selections were more resistant to root galling than the susceptible standards in two or more trials: PA98NM38-1 was more resistant than the susceptible standards in 5 of 5 trials, PO94A009-10 in 4 of 5 trials, PA95B2-4 and PA98N5-2 in 3 of 5 trials, POR00HG5-1 in 2 of 5 trials, PO94A009-7 in 3 of 4 trials, PO94A012-2 in 2 of 3 trials, and Summit Russet in 2 of 2 trials. POR00HG5-1 has Solanum hougasii in its ancestry, while the other selections have the Mexican wild species Solanum bulbocastanum and the commercial cultivar Summit Russet appearing in their ancestry. Summit Russet is the most plausible source of resistance.

Colonization of Potato Plants after Aerial Infection by Colletotrichum coccodes, Causal Agent of Potato Black Dot

Colonization of potato plants by Colletotrichum coccodes after artificial inoculation of stems was modeled in relation to the severity of chlorosis and necrosis on foliage using logistic regression. C. coccodes internally colonized the stems up to 24 cm above the inoculation court (mean of 11.3 ± 7.7 cm and 14.1 ± 9.3 cm in 2003 and 2004, respectively), and completely colonized the stem below the inoculation court including the roots (mean of 13.2 ± 3.1 cm and 12 ± 2.3 cm in 2003 and 2004, respectively). C. coccodes grew significantly faster from the inoculation court toward the roots, stolons, and tubers than toward the plant apex. Internal stem colonization was restricted before senescence but rapidly increased as the plants started to senescence, suggesting that physiological events associated with plant senescence activated fungal growth and plant colonization. The results showed no relationship between C. coccodes growth and plant colonization and the development of chlorosis and necrosis severity of leaves. The severity of chlorosis and necrosis on foliage in this study was the outcome of natural plant senescence, and was not caused by the fungal colonization. Therefore, chlorosis and necrosis severity is an inaccurate tool to diagnose infection in potato by C. coccodes and does not indicate the presence or amount of C. coccodes in potato plants. Disease assessment for C. coccodes in potato plants should not rely on severity of chlorosis and necrosis. Black dot assessment should be carried out by isolation and quantification of the fungus from stem and root tissues or by polymerase chain reaction techniques before plant senescence.

Genetic Variability in the Potato Pathogen Colletotrichum coccodes as Determined by Amplified Fragment Length Polymorphism and Vegetative Compatibility Group Analyses

ABSTRACT Amplified fragment length polymorphism (AFLP) using three primer sets was used to characterize 211 Colletotrichum coccodes isolates from North America, 112 of which were assigned to six vegetative compatibility groups (VCGs) using nitrate nonutilizing (nit) mutants. These isolates clustered into five corresponding groups by unweighted pairgroup method with arithmetic means-based cluster analysis of AFLP banding patterns. Isolates of C. coccodes belonging to NA-VCG1 and NA-VCG3 were closely related, as were isolates belonging to NA-VCG2 and NA-VCG5. Based on bootstrap analysis of AFLP data, the two isolates originally assigned to NA-VCG4 clustered with isolates belonging to NA-VCG2 and NA-VCG5. C. coccodes isolates that clustered with two isolates belonging to NA-VCG6 were the most diverged from other groups, including seven isolates collected from hosts other than potato. As opposed to the bootstrap analysis, a quadratic discriminant analysis (QDA) of AFLP data correctly categorized the two isolates of NA-VCG4. Furthermore, in isolates where VCG determinations had been made, this model correctly classified isolates of all VCGs. QDA classifications were identical to those made by the bootstrap analysis, with the exception of VCG4. Overall, classifications made by the QDA model were strongly correlated (r = 0.970, P < 0.001) to the VCGs assigned by traditional methods. All 99 C. coccodes isolates evaluated only by AFLP also were subjected to QDA, leading to the assignment of a presumptive VCG for each isolate. No isolates of VCG4 or VCG6 were identified by QDA within this population. Symptoms of black dot developed in plants inoculated with isolates collected from both potato and non-potato hosts. However, total yield was not significantly reduced by infection with non-potato isolates. The lack of any additional groups identified by AFLP analysis may be an indicator of a limited level of genetic variation among North American C. coccodes isolates. AFLP is a much more efficient technique for subspecific characterization in C. coccodes than VCG analysis utilizing nit mutants and will provide an effective means by which the population biology of this pathogen can be further investigated worldwide.

Disease Potential of Soil- and Tuberborne Inocula of Colletotrichum coccodes and Black Dot Severity on Potato

Inoculum of Colletotrichum coccodes, the cause of potato black dot, is soil- or tuberborne. Understanding the disease potential of sources of inocula is crucial for developing disease management strategies and resistance screening techniques. Two hypotheses were tested in this study: (i) soilborne inoculum causes more disease than tuberborne inoculum and (ii) black dot severity is related to the concentration of soilborne inoculum. Trials were conducted in the greenhouse with standardized inoculum. Plants grown in infested soil had more sclerotia on roots than plants grown from infected tubers in three of four trials. In general, plants grown in infested soil produced fewer tubers and lower yields than the noninoculated plants. Plants grown from infected tubers produced similar numbers of tubers as the noninoculated plants in all trials, and had reduced yields in one of the four trials. Increasing concentrations of soilborne inoculum had a nonlinear association with disease development. Foliar symptoms, sclerotial density on roots, and sclerotial development on stems did not increase when soil inoculum exceeded 0.5 or 1.7 g/liters of soil. In this study, soilborne inoculum caused more disease than tuberborne inoculum and disease severity remained constant above a threshold of soilborne inoculum.

Field Resistance to Potato Stem Colonization by the Black Dot Pathogen Colletotrichum coccodes

Potato (Solanum tuberosum) germplasm was tested for resistance to stem colonization by the black dot pathogen Colletotrichum coccodes. Forty-six potato selections were tested in three field trials from 2006 to 2008. Resistance was determined by comparing disease severity on aboveground stems to the mean disease severity of the industry standards Russet Burbank, Ranger Russet, and Umatilla Russet. The potato selections were also tested for genotype*environment interaction to determine their genetic stability. Heritability of resistance was calculated to be 0.13 with confidence intervals between 0.00 and 0.68. The selections A0012-5, PA95B2-4, PA98NM38-1, and PO94A009-7 had less black dot than the standards in all years, and also demonstrated genetic stability. These selections also possess resistance to the root galling stage of the powdery scab pathogen Spongospora subterranea f. sp. subterranea. PA95B2-4, PA98NM38-1, and PO94009-7 were derived from an introgression program to incorporate resistance to the Columbia root-knot nematode Meloidogyne chitwoodi from the Mexican wild species Solanum bulbocastanum, and also have the commercial cultivar Summit Russet in their ancestry. These selections are promising steps toward sustainable management of black dot and powdery scab and will be further tested and used for breeding purposes.

Hairy Nightshade is an Alternative Host of Spongospora subterranea, the Potato Powdery Scab Pathogen

Root galls possibly caused by Spongospora subterranea were observed on hairy nightshade (Solanum sarrachoide; HNS). HNS galls and galls from potato were used to artificially inoculate potato and HNS. Eighty-three and 52% potato and HNS plants inoculated with potato inoculum had root galls, respectively. Ten and 31% potato and HNS plants inoculated with HNS inoculum had root galls, respectively. A S. subterranea-specific PCR significantly correlated (P < 0.0001, Phi Coefficient = 0.69) with visual assessment of gall incidence in 34 of 40 samples. Five of 40 samples had no galls, but gave positive PCRs. One of 40 samples had root galls, but gave a negative PCR. More sporosori were produced on potato than on HNS. Therefore, the root galls on HNS were caused by S. subterranea that is able to complete its life cycle on HNS and produce new generations of sporosori that are infectious on potato.ResumenAgallas de la raíz, posiblemente causadas por Spongospora subterranea fueron observadas en belladona vellosa (Solanum sarrachoide; HNS). Agallas de HNS y de papa fueron utilizadas para inocular artificialmente papa y HNS. El 83% y el 52% de plantas de papa y HNS inoculadas con inóculo de papa tuvieron agallas de la raíz respectivamente. El 10% y 31% de plantas de papa y HNS inoculadas con inóculo de HNS tuvieron agallas de la raíz respectivamente. Un PCR especifico para S. subterranea correlacionó significativamente (P < 0.0001, Coeficiente Phi = 0.69) con evaluación visual de incidencia de agallas en 34 de 40 muestras. Cinco de 40 muestras no tuvieron agallas pero dieron PCR positivo. Una de las 40 muestras tuvo agallas de la raíz pero dio PCR negativo. Más sporosori fue producido en papa que en HNS. Por lo tanto las agallas de la raíz en HNS fueron causadas por S. subterranea que es capaz de completar su ciclo de vida en HNS y producir nuevas generaciones de sporosori que son infecciosas en papa.

Genetic Stability in Potato Germplasm for Resistance to Root Galling Caused by the Pathogen Spongospora subterranea

Spongospora subteranea, the causal agent of potato powdery scab is becoming increasingly important worldwide. Little is known about the genetic basis of resistance to this disease. The present study tested the hypothesis that potato genotypes with stable genetic resistance to “Spongospora root galling” were present in potato germplasm. Root galling index values of 24 genotypes screened for resistance in four field trials (environments) in 2004 and 2005 in Washington State and Idaho were analyzed. Genotypes tested included five resistant, four industry standards and advanced selections from the USDA-ARS, Prosser, WA program. Broad-sense heritability was calculated as 0.76 with a 95% confidence interval of 0.55–0.89, indicating a fairly high genetic component of the trait. Of the 24 genotypes that were tested, eight showed no genotype*environment interactions while six of the remainder had significant variance (i.e., they were unstable) after removal of genotype*environment variance. Among the five resistant genotypes, PA95B2-4 was stable, and PA98N5-2, PA98NM38-1, PO94A009-7 and POR00HG5-1 were stable after the removal of environmental heterogeneity. Among the four industry standards, Shepody was unstable, whereas Ranger Russet, Russet Burbank and Umatilla Russet were stable after the removal of genotype*environment variance. Stable resistance to “Spongospora root galling” was identified. A large portion of the variation was genetic, which will enable breeders to use resistant and stable potato genotypes as parents in future breeding to develop superior commercial potato cultivars with resistance to “Spongospora root galling”.ResumenSpongospora subteranea, el agente causal de la roña polvorienta se está convirtiendo de importancia en aumento en el mundo. Se sabe poco de las bases genéticas de la resistencia a esta enfermedad. El presente estudio probó la hipótesis de que los genotipos de papa con resistencia genética estable al “agallamiento radical por Spongospora” estaban presentes en germoplasma de papa. Se analizaron los valores del índice de agallamiento radical de 24 genotipos probados para resistencia en cuatro ensayos de campo (ambientes) en 2004 y 2005 en los Estados de Washington y Idaho. Los genotipos probados incluyeron cinco resistentes, cuatro estándares para la industria y selecciones avanzadas del programa USDA-ARS, Prosser, WA. Se calculó la heredabilidad en una base amplia como 0.76 con un intervalo de confianza de 95% de 0.55 a 0.89, indicando justamente un componente genético amplio del carácter. De los 24 genotipos probados, ocho no mostraron interacción genotipo-medio ambiente, mientras que seis del resto tuvieron varianza significativa (por ejemplo, fueron inestables) después de eliminar la varianza genotipo ambiente. Entre los cinco genotipos resistentes, PA95B2-4 fue estable, y PA98N5-2, PA98NM38-1, PO94A009-7 y POR00HG5-1 fueron estables después de eliminar la heterogeneidad del ambiente. Entre los cuatro genotipos estándares para la industria, Shepody fue inestable, mientras que Ranger Russet, Russet Burbank y Umatilla Russet fueron estables después de eliminar la varianza genotipo-ambiente. Se identificó resistencia estable a “agallamiento radical por Spongospora”. Una gran proporción de la variación fue genética, lo cual permitirá a los mejoradores usar genotipos resistentes y estables de papa como progenitores en mejoramiento futuro para desarrollar cultivares comerciales de papa superiores con resistencia al “agallamiento radical por Spongospora”.