Robert M. Harveson

Improving Root Health and Yield of Dry Beans in the Nebraska Panhandle with a New Technique for Reducing Soil Compaction

A field study conducted during the 2001 and 2002 growing seasons investigated the integration of fungicide applications and tillage methods for reducing root health problems in dry bean (Phaseolus vulgaris) plants by alleviating soil compaction and its potential exacerbation of root disease. Several cultural practices were combined with applications of the strobilurin fungicide azoxystrobin. Soil compaction was created artificially throughout the entire plot area. Six treatments, consisting of four tillage treatments and two combinations of tillage or applications of azoxystrobin, were tested to alleviate the compaction and enhance root health. Tillage treatments included a compacted control with no additional tillage, formation of beds approximately 10 cm above soil surface, zone tillage with an implement using in-row shanks, and both zone tillage and bedding combined. Fungicide treatments utilized the combination of both zone tillage and bedding with fungicide applications, and a fungicide treatment singly. Effects of compaction on plant vigor and disease development and severity were evaluated 67 and 83 days after planting in 2001 and 2002, respectively, by a visual estimation of plot vigor and by destructively sampling and making root and hypocotyl disease ratings on dry bean plants from nonharvest rows. Soil resistance and moisture were measured in plots 80 and 104 days after planting in 2001 and 2002, respectively, to estimate degree of compaction. In both years, Fusarium root rot, caused by Fusarium solani f. sp. phaseoli, was determined to be the main root disease impacting plant health in studies. All measured variables (root disease index, plant vigor ratings, total seed yield, seed size, and soil resistance) were significantly improved by any treatment that included zone tillage prior to planting. No added advantages were observed for decreasing disease or improving root health and plant performance with the use of azoxystrobin or by planting on raised beds. This is the first study to evaluate zone tillage as a method of reducing plant stress and root disease in dry bean plants.

Fungicide registration and a small niche market : A case history of hymexazol seed treatment and the U.S. sugar beet industry

The United States ranks among the top four sugar producers worldwide, and sugar beet (Beta vulgaris L.) plays a major role in the sweetener industry. Sugar beet was planted on approximately 553,100 ha (1.37 million acres) in 2006 (33). The greatest volume of production occurs in the Red River Valley (RRV) of Minnesota and North Dakota and in southern Minnesota. In 2005, this region planted 302,000 ha (746,000 acres) of sugar beet, which accounted for over half of the hectares sown (58%) and metric tons of roots produced (51%) in the United States (33); total economic impact of the crop exceeded

Bacterial Wilt of Dry-Edible Beans in the Central High Plains of the U.S.: Past, Present, and Future

3 billion (4). Three regions, including nine additional states, comprise the remainder of the production areas (Fig. 1). They include the Far West (California, Idaho, Oregon, Washington), Great Plains (Colorado, Montana, Nebraska, Wyoming), and Great Lakes (Michigan; Ohio stopped production in 2005), which produced 25, 13, and 12% of the total metric tons of roots harvested in 2005, respectively (33). Overall, the annual impact of producing and processing sugar beet in the United States contributes

New Outbreaks of Bacterial Wilt of Dry Bean in Nebraska Observed from Field Infections

4.5 billion to the economy and over 79,000 full-time equivalent jobs (30).

Occurrence and characterization of a new red-pigmented variant of Curtobacterium flaccumfaciens, the causal agent of bacterial wilt of edible dry beans in Iran

Bacterial wilt, caused by Curtobacterium flaccumfaciens pv. flaccumfaciens, was first recognized and described as a new dry bean disease near Redfield, SD after the 1921 growing season on the farm of the Office of Forage Investigations. Between the late 1930s and the early 1950s it became one of the more problematic bacterial diseases of dry beans. It became an endemic problem in dry bean production throughout western Nebraska and other areas of the central high plains during the 1960s and early 1970s. By the early 1980s, the disease had virtually disappeared with the implementation of cultural practices. The disease was rediscovered in two fields in Nebraska late in the 2003 season. It was assumed to be an isolated incident. However, the next season the pathogen was widespread throughout western Nebraska production fields. Our research suggests that the return of bean wilt throughout the central high plains over the last decade is not due to a single factor but a combination of new changes in cultural practices, environmental stresses, and unfamiliarity with the pathogen and its past history.

Utilizing a preplant soil test for predicting and estimating root rot severity in sugar beet in the central high plains of the United States

Bacterial wilt caused by Curtobacterium flaccumfaciens pv. flaccumfaciens was one of the more problematic diseases of dry bean (Phaseolus vulgaris L.) throughout the irrigated High Plains (Colorado, Nebraska, and Wyoming) in the 1960s and early 1970s, but has not been observed since that time. However, in August of 2003, plants exhibiting wilting and irregular, interveinal necrotic foliar lesions surrounded by a bright yellow border were found in three dry bean fields (market class Great Northern) in Scotts Bluff County, Nebraska. During 2004, plants exhibiting identical symptoms were additionally found occurring in more than 40 dry bean fields in western Nebraska. Affected fields were planted with dry bean from multiple market classes and seed sources, including yellow bean, Great Northern bean, and pinto bean, and incidence varied from trace levels to 80 to 90%. Isolations were made from leaf and stem tissues and seeds collected after harvest from infected plants, and all yielded slow-growing, creamy yellow or orange, fluidal colonies on nutrient broth-yeast extract medium. The bacterium was identified as C. flaccumfaciens pv. flaccumfaciens based on cell morphology (coryneformshaped motile rods), positive Gram stain and KOH reactions, fatty acid profiles, and BIOLOG (Hayward, CA) identifications. Great Northern (cv. Orion) plants were inoculated by bacterial suspensions (5 × 107 CFU/ml) injected into leaf axils adjacent to the first fully expanded trifoliolate and were incubated in the greenhouse under ambient conditions fluctuating between 24 and 35°C. Wilting symptoms developed 7 days after inoculation with foliar necrosis and yellowing symptoms appearing after 24 days. Identical bacterial colonies were reisolated from inoculated tissues, completing Kochs postulates. Although recent reports of wilt have been made in North Dakota (2) and western Canada (1) in 1995 and 2002, respectively, they were based only on the presence of discolored seeds observed in dockage from processing plants after harvest. To our knowledge, this report represents the first widespread observations of bacterial wilt from field infections in Nebraska in more than 30 years. References: (1) J. R. Venette et al. Plant Dis. 79:966, 1995. (2) T. F. Hsieh et al. Plant Dis: 86:1275, 2002.

First Report of Ascochyta Blight Caused by QoI-Resistant Isolates of Ascochyta rabiei in Chickpea Fields of Nebraska and South Dakota

A number of crop diseases are emerging at an alarming rate worldwide. Bacterial wilt of dry beans, caused by Curtobacterium flaccumfaciens pv. flaccumfaciens (Cff), is one of them. In Iran, this disease was first reported in 2012, which, since then, has rapidly spread across the major dry bean growing areas of the country causing severe yield losses. Previously, only two colony variants (yellow and orange) of the pathogen had been described from Iran in association with bacterial wilt of dry beans. In this study, we describe a new red-pigmented variant of Cff, isolated from dry bean seeds stored in seed banks of Khomein Bean Research Station, the major seed supplier in the region. Because Cff is a quarantine pathogen in Iran and elsewhere, with a potential threat for dry bean productions, more knowledge about the biology of this pathogen and epidemiology of the disease it causes are a prerequisite for the development of effective disease management strategies. Within this framework, we performed phenotypic and genetic characterization of the red-pigmented variant of the pathogen, in comparison with previously isolated yellow and orange variants, including pathogenicity, host range, bacteriocin production and genetic diversity. Our results showed a similar host range of different Cff variants although they differed in their aggressiveness. Yellow and orange variants of the pathogen were more aggressive on cowpea and common bean, respectively while the red variant showed the same level of aggressiveness on both hosts. Orange- and red-pigmented strains were separated from yellow-pigmented strains in the phylogeny of gyrB sequences. All orange- or red-pigmented strains were clustered in a separate branch from yellow-pigmented strains, except strain CffK31, in phylogeny based on rpoB sequences. In BOX-PCR analysis, Cff strains used in this study were clustered in two distinct genetic groups, with yellow variants of the pathogen separated from the orange and red variants. Overall, our results provide evidence of a remarkable diversity of Cff in Iran, which needs further in-depth investigation.

Phenotypic Diversity of Puccinia helianthi (Sunflower Rust) in the United States from 2011 and 2012

Aphanomyces cochlioides and Rhizoctonia solani are important soilborne pathogens causing root diseases that are primary constraints to sugar beet production in Nebraska, Colorado, and Wyoming. These types of diseases are difficult to control because they are often not noticed until substantial damage has already occurred. Efforts to manage them would be more effective if techniques were available that were more predictive than reactive. Therefore, a preplant soil test was developed to estimate the relative pathogen populations in the soil and to predict potential root disease problems later in the growing season. Preplant soil samples collected from fields to be sown with sugar beet were planted with a susceptible cultivar and tests were conducted for 1 month in the greenhouse. A preplant disease index was developed based on the time period during the test that seedlings became infected and was calculated on a 0-to-100 scale. Disease index values were compared with yields obtained from the same fields after harvest. Analysis of data collected for 5 years (2003 to 2007) with analysis of covariance revealed a strong relationship between the preplant disease index values and recoverable sucrose and root yields but not sucrose concentration. Results indicated that, for each unit increase in the preplant disease index, root yield decreased by 0.27 metric tons (270 kg) per hectare (P < 0.05, R2 = 0.44) and recoverable sucrose decreased by 49 kg/ha (P < 0.05, R2 = 0.45). We concluded that this preplant soil test can accurately predict root disease potential due to R. solani and A. cochlioides, and has the potential to help producers make effective management decisions in production fields using the index procedure. This soil assay has additionally provided new information on the biology, incidence, and distribution of root pathogens in production fields throughout the Central High Plains.

A Rare Epidemic of Sugar Beet Seedling Rust in Nebraska

Ascochyta blight, caused by Ascochyta rabiei, is a serious disease of chickpea (Cicer arietinum) and fungicide applications are used to manage the disease in the North Central plains (4). During the 2010 growing season, a commercial field near Stanley, SD was treated with pyraclostrobin (Headline, BASF, NC) and called a management failure by the grower. Similarly, limited efficacy of pyraclostrobin was observed in an ascochyta research trial near Scotts Bluff, NE. In both locations, symptoms and signs consistent with A. rabiei infection existed on leaves, stems, and pods; namely, circular brown lesions with concentric rings of dark brown pycnidia. Symptomatic samples were collected, disinfected with 95% ethanol for 1 min, rinsed with sterile water, placed in 0.5% NaOCl for 1 min, and rinsed again with sterile water for 1 min (4). Samples were air dried, placed on potato dextrose agar (PDA) plates for 3 to 7 days, and colonies with morphological characteristics typical of A. rabiei were single-spored and transferred to new PDA plates and incubated for 7 to 14 days. Three and six putative A. rabiei isolates were obtained from South Dakota and Nebraska samples, respectively. Morphological characteristics were consistent with A. rabiei; cultures were brown with concentric rings of dark, pear-shaped pycnidia with an ostiole, and conidia were hyaline, single-celled, and oval-shaped (2). Comparison of the internal transcribed spacer (ITS) region amplified from the genomic DNA of 3-day-old liquid cultures using ITS4/ITS5 primers by BLASTN searches using the nr database in GenBank (Accession Number FJ032643) also confirmed isolates to be A. rabiei. Mismatch amplification mutation assay (MAMA) PCR was used for detection of sensitive and resistant isolates to QoI fungicides (1). Confirmation of the presence of the G143A mutation was carried out by cloning an mRNA fragment of the cytochrome b gene using cDNA synthesized from total RNA of A. rabiei and CBF1/CBR2 (1,3). Total RNA was extracted from 3-day-old liquid cultures and it was used instead of genomic DNA for this PCR to avoid large intronic regions commonly present in mitochondrial genes. The G143A mutation has previously been correlated with resistance to QoI fungicides in other fungal plant pathogens (3). Also, these isolates were determined to be QoI-resistant in vitro by PDA amended with a discriminatory dose of 1 μg/ml of azoxystrobin (4). To our knowledge, this is the first report of QoIresistant A. rabiei isolates causing infections on chickpeas in South Dakota and Nebraska. QoI-resistant isolates were reported in North Dakota and Montana in 2005 and 2007, respectively (4). Of nearly 300 isolates collected from these states from 2005 and 2007, approximately 65% were determined to be QoI resistant (4). The widespread occurrence of QoIresistant isolates and reduction of fungicide performance in fields led the North Dakota State University Cooperative Extension Service to actively discourage the use of QoI fungicides on chickpeas in North Dakota and Montana (4). It is likely that similar recommendations will need to be adopted in South Dakota and Nebraska for profitable chickpea production. References: (1) J. A. Delgado, 2012 Ph.D. Diss. Department of Plant Pathology, North Dakota State University. (2) R. M. Harveson et al. 2011. Online. Plant Health Progress doi:10.1094/PHP-2011-0103-01-DG. (3) Z. Ma et al. Pestic. Biochem. Physiol. 77:66, 2003. (4) K. A. Wise et al. Plant Dis. 93:528, 2009.

Determining Optimal Fungicide Timing for Rhizoctonia Root Rot of Sugar Beet in Nebraska Based on Soil Temperatures

Puccinia helianthi, causal agent of sunflower rust, is a macrocyclic and autoecious pathogen. Widespread sexual reproduction of P. helianthi was documented in North Dakota and Nebraska for the first time in 2008 and has since frequently occurred. Concurrently, an increase in sunflower rust incidence, severity, and subsequent yield loss on sunflower has occurred since 2008. Rust can be managed with resistance genes but determination of virulence phenotypes is important for effective gene deployment and hybrid selection. However, the only P. helianthi virulence data available in the United States was generated prior to 2009 and consisted of aggregate virulence phenotypes determined on bulk field collections. The objective of this study was to determine the phenotypic diversity of P. helianthi in the United States. P. helianthi collections were made from cultivated, volunteer, and wild Helianthus spp. at 104 locations across seven U.S. states and one Canadian province in 2011 and 2012. Virulence phenotypes of 238 single-pustule isolates were determined on the internationally accepted differential set. In total, 29 races were identified, with races 300 and 304 occurring most frequently in 2011 and races 304 and 324 occurring most frequently in 2012. Differences in race prevalence occurred between survey years and across geography but were similar among host types. Four isolates virulent to all genes in the differential set (race 777) were identified. The resistance genes found in differential lines HA-R3 (R4b), MC29 (R2 and R10), and HA-R2 (R5) conferred resistance to 96.6, 83.6, and 78.6% of the isolates tested, respectively.