Carl A. Strausbaugh

Pathogenicity and real-time PCR detection of Fusarium spp. in wheat and barley roots

The pathogenicity of five Fusarium spp. frequently isolated from wheat and barley roots in southern Idaho was investigated during four growth-chamber experiments and two field studies. A real-time PCR assay for quantifying the presence of F. culmorum from infected root tissue was also developed based on nucleotide sequence for the tri5 gene. Fusarium culmorum, followed by F. acuminatum and F. reticulatum, resulted in the largest root lesions and percent infected root area. However, F. semitectum, followed by F. acuminatum and F. equiseti, had the greatest impact on total root length. The TaqMan-based real-time assay was able to quantify F. culmorum in root tissue from both growth-chamber and field studies down to 61 pg. The assay also detected F. pseudograminearum and F. graminearum but could not distinguish among these three Fusarium spp.

Survey of root diseases of wheat and barley in southeastern Idaho

Root diseases of wheat and barley in southeastern Idaho (Intermountain West area of the U.S.A.) have not been extensively studied. Surveys were conducted in 2001 and 2002 to assess root disease severity and to identify soilborne pathogens present in 81 wheat and 52 barley fields in 13 southeastern Idaho counties. Bipolaris sorokiniana and Fusarium culmorum were the most frequent pathogens isolated and also the most virulent in greenhouse pathogenicity tests. Other pathogenic fungi isolated from root lesions included Fusarium spp., Rhizoctonia solani, and Gaeumannomyces graminis var. tritici. Nematode soil assays revealed that 96% of the fields had lesion nematodes (mostly Pratylenchus neglectus) and 78% had stunt nematodes (Tylenchorhynchus spp.). Other nematode species were detected in less than 18% of the fields. The cereal cyst nematode (Heterodera avenae) was found in two fields. Greenhouse experiments with soil cores indicated that disturbing the soil to simulate disking reduced the severity of fusarium root rot. Plant breeders attempting to incorporate resistance to root rot into cereal crops in the Intermountain West should focus on screening with B. sorokiniana and F. culmorum isolates.

Curly top survey in the Western United States.

Curly top in sugar beet continues to be a challenging disease to control in the western United States. To aid in development of host resistance and management options, the curtovirus species composition was investigated by sampling 246 commercial fields along with nursery and field trials in the western United States. DNA was isolated from leaf samples and the species were identified using species-specific polymerase chain reaction primers for the C1 gene. Amplicons from 79 isolates were also sequenced to confirm identifications. Beet severe curly top virus (BSCTV) and Beet mild curly top virus (BMCTV) were widely distributed throughout the western United States, while only a few isolates of Beet curly top virus (BCTV) were found. In phylogenetic analysis, BSCTV, BMCTV, and BCTV isolates formed distinct groups in the dendrogram. Seven isolates not amplifiable with species-specific primers did amplify with curly top coat protein primers, indicating novel curtovirus species or strains may be present. Given the wide host range of the viruses responsible for curly top, frequent co-infections, and genetic diversity within and among species, establishing better host resistance, and controlling curly top will continue to be a challenge.

Virulence, distribution and diversity of Rhizoctonia solani from sugar beet in Idaho and Oregon

Abstract Rhizoctonia root rot causes serious losses on sugar beet worldwide. In order to help explain why Rhizoctonia root rot management practices have not performed well in some areas of the Intermountain West (IMW), a survey was conducted. In the IMW from 2004 to 2006, 94 Rhizoctonia solani field isolates were collected from sugar beet roots. These field isolates were compared with 19 reference strains and 46 accessions from GenBank for genetic diversity based on sequencing of the ITS-5.8S rDNA region. Greenhouse pathogenicity tests on sugar beet and silage corn were conducted and plant damage was assessed using a randomized complete block design with at least four replications. The majority of the isolates had sequence identity with the AG-2-2 IIIB (47%) or AG-4 subgroups (44%). Most of the AG-2-2 isolates (87%) were associated with fields in the western portion of the production area, while 71% of the AG-4 isolates came from the eastern portion of the production area. Isolates from AG-2-2 IIIB were frequently more virulent on sugar beet and sequence of the ITS-5.8s region required cloning because of genetic diversity within isolates. Seven (all AG-2-2 IIIB) of 18 isolates tested could attack both sugar beet and corn, while two of the seven virulent isolates caused less root rot on corn. To reduce Rhizoctonia root rot on sugar beet and corn, crop rotations and the isolates utilized for selecting host resistance could be given further consideration.

Seed Transmission of the High Plains virus in Sweet Corn

The High Plains virus (HPV), which infects corn and other cereals, was first found in 1993 in the United States. Research was initiated in 1995 to investigate the potential for seed transmission of HPV. Sweet corn seeds of various cultivars harvested in 1994 to 1996 from 13 fields and research plots in southwestern Idaho, Colorado, and Nebraska were seeded in potting mix in the greenhouse. Leaf samples collected at the three- to six-leaf stage from both symptomatic and asymptomatic plants were tested by enzyme-linked immunosorbent assay (ELISA). Of the 46,600 seeds planted, 38,473 seedlings emerged, and three tested positive by ELISA, exhibited mosaic symptoms, and had the presence of HPV confirmed by an additional test. One of the positive plants was used for successful acquisition and transmission of HPV by the wheat curl mite to Westford barley. The other two plants were used to successfully transfer HPV to other corn plants by vascular puncture inoculation of seed. These results indicate that HPV can be seed transmitted at a very low frequency in sweet corn.

Bacteria and Yeast Associated with Sugar Beet Root Rot at Harvest in the Intermountain West

An undescribed wet rot of roots was observed in surveys of recently harvested sugar beet roots in Idaho and eastern Oregon in 2004 and 2005. Microorganisms isolated from 287 roots fell into the following groups: A (41% of strains), B (29%), C (17%), D (11%), E (2%), and F (1%). Groups A, B, C, and F were composed of bacteria while groups D and E were yeasts. Subgroup A1 (80% of group A strains) included Leuconostoc mesenteroides subsp. dextranicum strains and subgroup A2 (20%) contained Lactobacillus strains. Group B was dominated by subgroup B1 (92% of strains), which included Gluconobacter strains. When only one organism was isolated from rotted roots, strains from subgroup A1 were isolated most frequently. Group C was composed of enteric bacteria. Strain B322 of L. mesenteroides subsp. dextranicum caused the most severe rot on root slices and produced symptoms similar to those in harvested roots. Results suggest that L. mesenteroides subsp. dextranicum is among the first bacterial species to enter sugar beet roots, closely following fungal infections or entering directly through openings such as growth cracks. The bacterial rot leads to yield loss in the field but likely also leads to storage and factory-processing problems.

Genetic Characterization of Differential Reactions Among Host Group 3 Common Bean Cultivars to NL-3 K Strain of Bean common mosaic necrosis virus

ABSTRACT A previously unrecognized recessive resistance gene (or allele) was identified in three host group (HG) 3 common bean (Phaseolus vulgaris) cvs. Olathe, Victor, and UI 37, based on genetic analysis of plants from five populations screened with the NL-3 K strain of Bean common mosaic necrosis virus (BCMNV). The gene (or allele) was associated with resistance to leaf stunting and deformity and reduction in plant height. The gene (or allele) provides similar, but slightly better resistance than the bc-1(2) gene that is characteristic of HG 3 cultivars. Traditional HG 3 cultivars like Redlands Greenleaf B with bc-1(2) are susceptible to NL-3 K, whereas this newly identified gene (or allele) conditions resistance to NL-3 K. Other slight variations in disease reaction pattern to a wide array of bean common mosaic (BCM)-inducing strains were noted among HG 3 differentials, indicating that additional resistance to BCM exists in common bean that remains to be exploited. To gauge the full breeding value of this newly identified gene (or allele), allelism tests with existing genes, namely bc-1(2), and further characterization of responses to all Bean common mosaic virus (BCMV) and BCMNV strains need to be conducted. Meanwhile, breeders should consider introgressing this more effective gene (or allele) into susceptible cultivars while plant pathologists continue to decipher the genetic variability present among HG 3 differential cultivars.

Influence of Host Resistance and Insecticide Seed Treatments on Curly Top in Sugar Beets

Curly top on sugar beets (Beta vulgaris) caused by Beet severe curly top virus or closely related species is a considerable problem in arid growing regions of the western United States. Two insecticide seed treatments, Poncho Beta (60 g a.i. clothianidin + 8 g a.i. beta-cyfluthrin/100,000 seed) and Gaucho (45 g a.i. imidacloprid/100,000 seed), and four sugar beet hybrids varying in curly top resistance were evaluated for their influence on the control of curly top in comparison with untreated checks. Plots were established at two locations in southern Idaho in 2005 and evaluated for curly top. Moderate to severe curly top due to natural inoculum and leafhopper infestations occurred at both locations. Untreated, the four hybrids performed as expected with the fewest curly top symptoms on PM21 and the most on Monohikari. Both insecticide treatments lowered curly top ratings compared with the untreated check, but Poncho Beta reduced symptoms more than Gaucho as the season progressed. Poncho Beta led to increased yield and estimated recoverable sugar across all hybrids at harvest, particularly on the more susceptible hybrids. When considering the yield parameters for only the most resistant hybrids individually, Poncho Beta did not always outperform Gaucho. Poncho Beta provided a level of control that would justify its application as a supplement to host resistance under Idaho conditions.

Relationship of Beet Curly Top Foliar Ratings to Sugar Beet Yield

Sugar beet (Beta vulgaris) varieties were evaluated for disease resistance to curly top to establish if disease ratings made in inoculated nurseries correlated with disease ratings and yield in sugar beet crops exposed to natural disease outbreaks. Cultivars were planted both in inoculated curly top nurseries in Kimberly, ID, and in commercial cultivar trials in irrigated fields near Ontario, OR and Nampa, ID. Plants were evaluated for curly top using a rating scale of 0 (no symptoms) to 9 (dead). Moderate disease pressure in the Ontario (mean rating = 3.8) and Nampa (mean rating = 4.1) fields resulted in significant differences for disease rating, root yield, sugar content, and estimated recoverable sugar among cultivars. Disease ratings from both commercial fields were positively correlated (r = 0.91 and 0.82, P < 0.0001) with ratings from the inoculated nurseries. In commercial fields, root yield was negatively related to disease rating (r2 = 0.47 and 0.39, P ≤ 0.0004). For each unit increase in disease rating (increasing susceptibility), root yield decreased 5.76 to 6.93 t/ha. Thus, curly top nurseries reliably predict curly top resistant cultivars for commercial cultivation.

Sugar beet root rot at harvest in the US Intermountain West

Root rot in sugar beet (Beta vulgaris) causes significant losses worldwide. To assess the distribution of root rot fungi and their relationship to bacterial root rot, commercial sugar beet roots with rot symptoms were collected at harvest time in the Intermountain West. Isolations for both fungi and bacteria were conducted using standard microbiological techniques, and the root area rotted was assessed. A subset of fungal isolates was tested for pathogenicity to sugar beet in greenhouse assays and field trials with and without manure. In the field survey of rotting beets, the fungi most frequently associated with root rot included Fusarium spp. (Fusarium oxysporum and Fusarium acuminatum with 24% and 15% of isolates, respectively), Geotrichum spp. (16% of isolates), Rhizoctonia solani (15% of isolates), and Mucor spp. (14% of isolates). However, only R. solani isolate F321 (AG-2-2IIIB) consistently caused rot in greenhouse pathogenicity tests. In the field survey, a mean of 6% of the root tissue had rotted for individual roots when fungi were isolated individually, whereas mean root rot was 71% and 68% when bacteria were isolated individually or in combination with other organisms, respectively. In field trials, roots inoculated with F321 averaged 3%–5% fungal rot, whereas the percentage of root tissue with bacterial rot was 6%–78%, which supports survey observations. Manure did not lead to root rots in the field. Traditionally, fungal root rots have been the main focus of breeding programs; however, because of the root area rotted by lactic acid bacteria, especially Leuconostoc, these bacteria should not be ignored in breeding efforts.