P. S. Wharton

Optimizing Fungicide Timing for the Control of Rhizoctonia Crown and Root Rot of Sugar Beet Using Soil Temperature and Plant Growth Stages

Azoxystrobin is applied early in the sugar beet growing season in north-central United States for control of Rhizoctonia damping-off and Rhizoctonia crown and root rot caused by Rhizoctonia solani anastomoses groups (AGs) 4 and 2-2, respectively. Fungicide application timings based on crop growth stage and soil temperature thresholds were evaluated in inoculated small-scale trials and in commercial fields with a history of Rhizoctonia crown and root rot. Soil temperature thresholds of 10, 15, and 20°C were selected for fungicide application timings and used to test whether soil temperature could be used to better time applications of azoxystrobin. In both small- and large-plot trials, timing applications after attainment of specific soil temperature thresholds did not improve efficacy of azoxystrobin in controlling damping-off or Rhizoctonia crown and root rot compared with application timings based on either planting date, seedling development, or leaf stage in a susceptible (E-17) and a resistant (RH-5) cultivar. Application rate and split application timings of azoxystrobin had no significant effect on severity of crown and root rot. Other environmental factors such as soil moisture may interact with soil temperature to influence disease development. Cv. RH-5 had higher sugar yield attributes than the susceptible cultivar (E-17) in seasons conducive and nonconducive to crown and root rot development. All isolates recovered from both small- and large-plot trials in all years were AG 2-2. R. solani AG 4 was not identified in any samples from any year.

First Report of In-Vitro Boscalid-Resistant Isolates of Alternaria solani Causing Early Blight of Potato in Idaho

Early blight of potato (Solanum tuberosum) is caused by Alternaria solani and occurs annually to some degree in Idaho. The timing of its appearance and rate of disease progress determine the impact on the potato crop. Though losses rarely exceed 20%, they can be higher and occur in stored potatoes if the disease is not controlled. Early blight is managed mainly by cultural practices such as plant nutrition, water management, and avoidance of plant stress, but also with the use of foliar fungicides. Currently, the main fungicides labeled for control of early blight are in the carboxamide and strobilurin fungicide groups. Development of resistance to some fungicide groups may contribute to the loss of control of early blight. Isolates of A. solani from Idaho potatoes were tested for resistance to boscalid in the carboxamide group. Diseased potato leaves with early blight symptoms were collected from fields near Parma, Rupert, and Aberdeen in southern Idaho in 2009 and 2010 and Bonners Ferry in northern Idaho in 2010. To obtain A. solani isolates from leaves, small pieces of leaf tissue (5 × 5 mm) were taken from the center of early blight lesions and streaked across the surface of a thin layer (3 mm) of water agar. Plates were incubated at 25°C overnight to allow spores to germinate. Single germinated A. solani spores were transferred to acidified potato dextrose agar and incubated in the dark at 25°C. Germinated spores were identified based on spore morphology. Spores of A. solani can be easily distinguished from other Alternaria spp. found on potato because they are ellipsoid to oblong and taper to a long beak that is usually as long as the spore body. The identity of cultures grown from single spores was confirmed by colony and spore morphology. Sensitivity of A. solani isolates to boscalid was determined by the spiral gradient endpoint method (2). For all isolates, the effective concentration for 50% reduction in growth was outside the range of the spiral plate dilution series (i.e., isolates were either completely insensitive or completely sensitive to boscalid). In total, 46 isolates (20 collected in 2009 and 26 collected in 2010) were tested against boscalid. Experiments were carried out twice with 2009 isolates using mycelial strips and conidial suspensions. Experiments with 2010 isolates were carried out three times using only conidial suspensions. Of the isolates from 2009, 15% were insensitive. There was no difference between the use of mycelial strips or conidial suspensions. In 2010, 62% of isolates were insensitive. By location, 72% of isolates from Parma, 73% from Rupert, 63% from Aberdeen, and 44% from Bonners Ferry were insensitive. Resistance to boscalid has been reported in A. alternata isolates from pistachio (1,3). However, to our knowledge, this is the first report of resistance to boscalid in isolates of A. solani on potato. These data suggest that resistance to boscalid is widespread in Idaho, even in areas like Bonners Ferry where potato cultivation is limited. Boscalid insensitivity in vitro may not translate directly to commercial production and currently there is no evidence to suggest that boscalid has failed to control early blight in Idaho. However, the discovery of insensitive isolates suggests that boscalid should be considered at high risk of resistance development. References: (1) H. Avenot et al. Plant Dis. 91:1345, 2007. (2) H. Förster et al. Phytopathology 94:163, 2004. (3) N. Rosenzweig et al. (Abstr.) Phytopathology 93(suppl.):S75, 2003.

Seed treatment application-timing options for control of fusarium decay and sprout rot of cut seedpieces

The efficacy of application timings of a commercially formulated mixture of fludioxonil plus mancozeb (Maxim MZ) applied prior to planting for the control of seedpiece decay and rotting of sprouts, caused by the dry rot pathogenFusarium sambucinum, was evaluated over two years. Cut potato seedpieces were inoculated with a virulent strain ofF. sambucinum and either treated with the fungicide mixture or not. Treatment applications were made 10, 5 or 2 days prior to planting. Seedpiece and sprout health were evaluatedin vitro and agronomic impacts were evaluatedin vivo in field experiments. Overall, thein vitro experiments indicated that inoculation withF. sambucinum did not have an effect on the mean number of sprouts per seedpiece but did affect the incidence of rotting sprouts and seedpiece decay. However, treatment of seedpieces with the fungicide mixture 10, 5 or 2 days before planting significantly reduced the percentage of diseased sprouts per seed-piece and seedpiece decay. Inoculated seedpieces treated with the fungicide mixture produced similar numbers of healthy sprouts as did the non-inoculated seedpieces. The experiment, conducted in 2003, showed that final plant stand, RAUEPC and RAUCCC were similar for non-inoculated seedpieces and inoculated fungicide-treated seedpieces. Overall, there were no significant differences among treatment timings, and these results suggest that applying a fungicide seed treatment up to 10 days prior to planting can provide effective control of both Fusarium seedpiece decay and sprout rot.ResumenDurante dos años se evaluó el mejor tiempo de aplicación de una mezcla formulada comercialmente de fludioxonil mas mancozeb (Maxim MZ) aplicada antes de la siembra para el control de la descomposición de semilla y pudrición de brotes, causada por el patógeno de la pudrición secaFusarium sambucinum. Semilla cortada de papa tratada y no tratada con la mezcla fungicida fue inoculada con un strain virulento deF. sambucinum. Las aplicaciones del tratamiento se hicieron 10, 5 o 2 días antes de la siembra. La sanidad de la semilla cortada y de los brotes fue evaluadain vitro y el impacto agronómico se evaluóin vivo. En términos generales, los experimentosin vitro indicaron que la inoculación conF. sambucinum no tuvo efecto sobre el promedio de brotes por semilla cortada, pero si afectó la incidencia de pudrición de brotes y de descomposición de la semilla. Sin embargo, el tratamiento de la semilla con la mezcla fungicida 10, 5 o 2 días antes de la siembra, redujo significativamente el porcentaje de brotes enfermos por semilla y la pudrición de la semilla. Las semillas tratadas con la mezcla fungicida produjeron un número similar de brotes sanos que la semilla no inoculada. El experimento hecho el 2003, mostró que el resultado final de plantas, RAUEPC y RAUCCC fueron similares entre las semillas inoculadas tratadas con el fungicida y las no inoculadas. En términos generales no hubo diferencia significativa entre el tiempo de aplicación de los tratamientos y estos resultados sugieren que aplicando un fungicida como tratamiento de semilla 10 días antes de la siembra puede proporcionar un control efectivo para Fusarium tanto para la descomposición de la semilla como para la pudrición del brote.

Effect of Temperature on Apothecial Longevity and Ascospore Discharge by Apothecia of Monilinia vaccinii-corymbosi

Pseudosclerotia of Monilinia vaccinii-corymbosi with apothecial initials were collected from blueberry (Vaccinium corymbosi cv. Jersey) fields in Grand Junction, MI, in the spring of 2002 and 2003 and monitored during apothecial development and maturation. Apothecia with cup openings as small as 1.2 mm in diameter were able to discharge ascospores. The number of ascospores discharged increased exponentially as the diameter of the cup opening increased, before leveling off at a cup diameter of about 6 mm. Cytological studies showed that ascospore discharge correlated well with the stage of development of asci and ascospores in the apothecium. The effect of temperature on the longevity of apothecia was studied at 10, 15, 20, and 25°C. Initial ascospore discharge was greater for apothecia incubated at higher temperatures (20 and 25°C). However, total discharge was greater in apothecia incubated at lower temperatures (10 and 15°C), as they persisted longer than those incubated at 20 and 25°C. Incubating apothecia at freezing temperatures (-2, -4, -6, and -8°C) for 1 h resulted in a 50 to 98% reduction in the number of ascospores discharged compared with before freezing. However, apothecia incubated down to -6°C were able to recover their ability to discharge ascospores by 2 days after the freezing treatment. Ascospores discharged from apothecia after freezing at -2°C for 1 h remained viable. However, the germination ability of ascospores from apothecia frozen at -4, -6, and -8°C decreased with decreasing temperature such that only 10% of ascospores from apothecia frozen at -8°C germinated successfully. The information obtained in this study may be useful in the development of scouting and management strategies for the control of mummy berry disease.

First Report of Rhizoctonia solani AG4 HG-II Infecting Potato Stems in Idaho

The fungus Rhizoctonia solani is the causal agent of stem canker and black scurf of potato (Solanum tuberosum). R. solani is a species complex consisting of 13 anastomosis groups (AGs) designated AG1 to 13 (2, 3). Stems of potato (cv. Russet Norkotah) with brown lesions were recovered from one field in Kimberley, Idaho, in August 2011. Using previously described methods (3), R. solani was recovered from the symptomatic stems and one representative isolate (J15) was selected for further characterization. Sequencing of the rDNA ITS region of isolate J15 was undertaken as previously described (3) and the resulting rDNA ITS sequence (HE667745) was 99% identical to sequences of other AG4 HG-II isolates in GenBank (AF354072 and AF354074). Pathogenicity of the isolate was determined by conducting the following experiment. Mini-tubers of cv. Santé were planted individually in 1-liter pots containing John Innes Number 3 compost (John Innes Manufacturers Association, Reading, UK). Pots were either inoculated with J15, an isolate of AG3-PT (Rs08), or were not inoculated. Each treatment was replicated four times. Inoculum consisted of five 10-mm-diameter potato dextrose agar plugs, fully colonized by the appropriate isolate, placed in the compost approximately 40 mm above each seed tuber. Pots were held in a controlled environment room at 21°C with 50% relative humidity and watered as required. After 21 days, plants were assessed for disease. No symptoms of the disease were present in non-inoculated plants. In the Rs08 (AG3-PT) inoculated plants, all stems displayed large brown lesions and 20% of the stems had been killed. No stem death was observed in J15 (AG4 HG-II) inoculated plants. However, brown lesions were observed in three of the four J15 (AG4 HG-II) inoculated plants. These lesions were less severe than in plants inoculated with the Rs08(AG3-PT) inoculated plants and were present in 40% of the main stems. In the J15 (AG4 HG-II) inoculated pots, R. solani AG4 HG-II was reisolated from the five symptomatic stems, thereby satisfying Kochs postulates. To our knowledge, this is the first report of AG4 HG-II causing disease on potatoes in Idaho. AG4 has been isolated from potato previously from North Dakota, although the subgroup was not identified (1). The only previous report where AG4 HG-II was specifically determined to cause disease on potato was in Finland, but the isolate could not be maintained and Kochs postulates were not completed (3). The present study shows that AG4 HG-II can cause stem disease in potatoes, although disease does not develop as severely or as consistently as for AG3-PT. However, as demonstrated with isolates of AG2-1 and AG5, even mild stem infection can reduce tuber yield by as much as 12% (4). AG4 HG-II is a pathogen of sugar beet in Idaho, which was grown previously in this field. This history may have contributed to high levels of soilborne inoculum required to produce disease on potato. References: (1) N. C. Gudmestad et al. Page 247 in: J. Vos et al. eds. Effects of Crop Rotation on Potato Production in the Temperate Zones. Kluwer, Dordrecht, Netherlands, 1989. (2) M. J. Lehtonen et al. Agric. Food Sci. 18:223, 2009. (3) J. W. Woodhall et al. Plant Pathol. 56:286, 2007. (4) J. W. Woodhall et al. Plant Pathol. 57:897, 2008.

First report of Rhizoctonia solani AG2-2IIIB infecting potato stems and stolons in the United States.

Rhizoctonia solani is an important pathogen of potato (Solanum tuberosum) causing qualitative and quantitative losses. It has been associated with black scurf and stem canker. Isolates of the fungus are assigned to one of 13 known anastomosis groups (AGs), of which AG3 is most commonly associated with potato disease (2,4). In August 2011, diseased potato plants originating from Rupert, ID (cv. Western Russet) and Three Rivers, MI (cv. Russet Norkotah) were received for diagnosis. Both samples displayed stem and stolon lesions typically associated with Rhizoctonia stem canker. The presence of R. solani was confirmed through isolation as previously described (4) and the Idaho and Michigan isolates were designated J11 and J8, respectively. AG was determined by sequencing the rDNA internal transcribed spacer (ITS) region using primers ITS5 and ITS4 (3). The resulting sequences of the rDNA ITS region of isolates J8 and J11 (GenBank Accession Nos. HE608839 and HE608840, respectively) were between 97 and 100% identical to that of other AG2-2IIIB isolates present in sequence databases (GenBank Accession Nos. FJ492075 and FJ492170, respectively). Kochs postulates were confirmed for each isolate by carrying out the following protocol. Each isolate was cultured on potato dextrose agar for 14 days. Five 10-mm agar plugs were then placed on top of seed tubers (cv. Maris Piper) in 1-liter pots containing John Innes Number 3 compost (John Innes Manufacturers Association, Reading, UK). Pots were held in a controlled environment room at 18°C with 50% relative humidity and watered as required. After 21 days, plants were removed and assessed for disease. Typical Rhizoctonia stem lesions were observed and R. solani was successfully reisolated from symptomatic material. To our knowledge, this is the first report of AG2-2IIIB causing disease on potatoes in the United States. In the United States, AGs 2-1, 3, 4, 5, and 9 have all been previously implicated in Rhizoctonia potato disease (2). AG2-2IIIB should now also be considered a potato pathogen in the United States. Knowledge of which AG is present is invaluable when considering a disease management strategy. AG2-2IIIB is a causal agent of sugar beet (Beta vulgaris) root rot in Idaho (1). Sugar beet is commonly grown in crop rotation with potato and such a rotation could increase the risk of soilborne infection to either crop by AG2-2IIIB. References: (1) C. A. Strausbaugh et al. Can. J. Plant Pathol. 33:210, 2011. (2) L. Tsror. J. Phytopatol. 158:649, 2010. (3) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, Inc., New York, 1990. (4) J. W.Woodhall et al. Plant Pathol. 56:286, 2007.

Effect of Soil Salinity on Internal Browning of Potato Tuber Tissue in Two Soil Types

A study was carried out with potato (Solanum tuberosum; cv. Atlantic) during 2001 and 2002 to determine the effect of soil salinity on internal tuber browning. The effect of varying levels of soil salinity on proline content, polyphenol oxidase enzyme activity, and chlorogenic acid content in potato leaves and tubers was examined. NaCl treatments (2.1, 4.25, 6.38, 8.5 g NaCl L−1) were applied to the pots, the first 46 days after planting, and four additional treatments were applied, each about 7 days apart. Increasing NaCl concentrations resulted in an increase in browning of tuber tissue and proline content in the tubers. Chlorogenic acid content in the leaves increased up to 6.4 g NaCl L−1, but then decreased at 8.5 g NaCl L−1 and in tubers tended to be maximal at the highest saline concentration tested (8.5 g NaCl L−1). Increasing NaCl concentration resulted in a reduction in yield per plant and average tuber weight, and also increased tuber number. There were major differences in the impact of salinity over the 2-year period, which was probably due to the impact of the growing media; a low organic matter (about 1% OM) silty loam soil and a high organic content (about 90% OM) Muck soil were used in 2001 and 2002, respectively. Tuber browning increased linearly with salinity in 2002, but only markedly increased at 8.5 g NaCl L−1 in 2001. Sodium and chloride ion concentration was always greater (about two times) at equivalent application rates in the 2002 trial. The high organic matter content soil retained sodium and chloride ions more effectively than the silty loam soil and enhanced the impact of increased salinity concentration on physiological properties of potato plants and particularly on tuber tissue browning and proline accumulation.ResumenCon el objeto de determinar el efecto de la salinidad del suelo sobre el oscurecimiento interno del tubérculo de papa (Solanun tuberosum; cv. Atlantic), se hizo un estudio durante los años 2001 y 2002. se examinó el efecto de la variación de niveles de salinidad del suelo sobre el contenido de prolina, actividad de la enzima polifenol oxidasa y contenido de ácido clorogénico en las hojas y tubérculos de papa. Los primeros 46 días después de la siembra se aplicó a las macetas los tratamientos de ClNa (2.1, 4.25, 6.38, 8.5g de ClNa L−1) y 4 tratamientos adicionales cada 7 días. El incremento de concentración de ClNa dio como resultado un aumento del oscurecimiento del tejido del tubérculo. El contenido de ácido clorogénico en las hojas aumentó hasta los 6.4g de ClNa L−1 pero luego disminuyó a los 8.5g y los tubérculos propendieron a tener mayor tamaño a las más altas concentraciones salinas probadas (8.5g ClNa L−1). El incremento de la concentración de ClNa dio como resultado una reducción del rendimiento por planta y del peso promedio del tubérculo, pero aumentó el número de tubérculos. Hubo diferencias importantes en el impacto de la salinidad sobre el período de los dos años, lo cual probablemente fue debido al impacto de los medios de cultivo, uno con baja cantidad de materia orgánica (alrededor de 1%) suelo fangoso con arcilla y un suelo con alto contenido de materia orgánica (alrededor de 90%) con estiércol húmedo, suelos que se usaron en 2001 y 2002 respectivamente. El oscurecimiento del tubérculo se incrementó linealmente con la salinidad en el 2002, pero solo aumentó marcadamente con 8.5g ClNa L−1 en el 2001. La concentración de los iones cloro y sodio fue siempre mayor (aproximadamente dos veces) a índices de aplicación equivalentes en la prueba del 2002. El suelo con alto contenido de materia orgánica retuvo los iones de cloro y sodio con mayor efectividad que el suelo fangoso arcilloso e intensificó el impacto de la concentración de salinidad sobre las propiedades fisiológicas de las plantas de papa y particularmente en el oscurecimiento del tejido y acumulación de prolina.

Improved weather-based late blight risk management: Comparing models with a ten year forecast archive

Agroecosystem decision support systems typically rely on some types of weather data. Although many new digital weather and forecast datasets are gridded data, the current authors feel that evaluating previous methods with data of increased archive length is critical in aiding the transition to new datasets that lack extensive archives. To that end, the present paper reviews the improvements made to an artificial neural network for forecasting weather-based potato late blight (Phytophthora infestans) risk at 26 locations in the Great Lakes region. Accuracies of predictions made using an early model, developed in 2007, are compared with accuracies of predictions made using a new 10-year hourly optimized model. In nearly every comparison by month, forecast lead time and spatial region, the newly optimized model is more accurate, especially when the weather is conducive to high disease levels.

Rhizoctonia solani Anastomosis Group 3 is Predominant in Potato Tubers in Poland

Rhizoctonia solani is a species complex of 13 related but genetically distinct anastomosis groups (AGs). In potato, R. solani can infect the stems, stolons, and roots, resulting in quantitative losses. It can also cause qualitative losses through blemishes occurring on progeny tubers, such as black scurf and elephant hide (corky cracking). Knowledge of the AG in local populations is important because they differ in host range, fungicide sensitivity, and disease severity (2). To determine the AGs present in Poland, 54 tuber samples displaying typical R. solani symptoms were taken from six different fields in 2011. The fields were representative of five different administrative regions of Poland and from at least 10 different varieties. Rhizoctonia was isolated from tubers by placing symptomatic material on to tap water agar amended with streptomycin and penicillin and after 2 to 3 days Rhizoctonia colonies were identified and hyphal tips of these transferred to potato dextrose agar. Rhizoctonia was successfully isolated from 48 tubers displaying black scurf and two tubers displaying elephant hide symptoms. DNA was extracted from Rhizoctonia cultures using a Wizard Food kit (Promega) and the AG was determined using specific real-time PCR assays (1). All Rhizoctonia isolates were determined to be AG3 and this was confirmed for 10 selected isolates by observing hyphal fusion with a known AG3 tester isolate (Rs08) as described previously (3). Pairings were also conducted amongst the 10 Polish isolates, C2 reactions were typically observed indicating numerous vegetative compatible groups are present. This study shows that AG3 is likely to be the predominant AG in potato tubers in Poland. This is similar to other studies in Europe, which have all determined that AG3 accounts for at least 92% of isolates from potato (2,3). AG2-1, 4, and 5 have also been found in tubers worldwide and climate and certain crop rotations can influence the presence of these other AGs in potato tubers (2). However, climate and crop rotations in Poland are similar to other parts of Europe so the predominance of AG3 is expected. AG3 was also isolated from elephant hide symptoms; however, it was more frequently isolated from sclerotia. The ability of AG3 to prolifically produce sclerotia and thereby survive on seed tubers may explain its predominance in potato crops (4). Therefore, studies focusing on the management of Rhizoctonia potato disease in Poland should consider AG3 in the first instance. References: (1) G. E. Budge et al. Plant Pathol. 58:1071, 2009. (2) L. Tsror. J. Phytopathol. 158:649, 2010. (3) J. W. Woodhall et al. Plant Pathol. 56:286, 2007. (4) J. W. Woodhall et al. Plant Pathol. 57:5, 2008.