Barry J. Jacobsen

Integrated Control of Rhizoctonia Crown and Root Rot of Sugar Beet with Fungicides and Antagonistic Bacteria

Rhizoctonia crown and root rot, caused by the fungus Rhizoctonia solani AG 2-2, is one of the most damaging sugar beet diseases worldwide and causes significant economic losses in more than 25% of the sugar beet production area in the United States. We report on field trials in the years 1996 to 1999 testing both experimental fungicides and antagonistic Bacillus sp. for their potential to reduce disease severity and increase sugar yield in trials inoculated with R. solani AG 2-2. Fungicides were applied as in-furrow sprays at planting or as band sprays directed at the crown at the four-leaf stage, or four- plus eight-leaf stage, while bacteria were applied at the four-leaf stage only. The fungicides azoxystrobin and tebuconazole reduced crown and root rot disease by 50 to 90% over 3 years when used at rates of 76 to 304 g a.i./ha and 250 g a.i./ha, respectively. The disease index at harvest was reduced and the root and sugar yield increased with azoxystrobin compared with tebuconazole. The combination of azoxystrobin applied at 76 g a.i./ha and the Bacillus isolate MSU-127 resulted in best disease reduction and greatest root and sucrose yield increase.

Oxidative burst elicited by Bacillus mycoides isolate Bac J, a biological control agent, occurs independently of hypersensitive cell death in sugar beet.

Response of sugar beet cultivars C40 and USH11 to syringe infiltration of live and dead Bacillus mycoides isolate Bac J, a biological control agent, and virulent and avirulent isolates of Erwinia carotovora pv. betavasculorum was measured by monitoring systemic acquired resistance control of Cercospora beticola, specific activity of chitinase and beta-glucanase, the oxidative burst, and hypersensitive cell death at the infiltration site. Priming sugar beet with B. mycoides Bac J (1 x 10(8) cells/ml) and avirulent isolates of E. carotovora pv. betavasculorum (1 x 10(6) cells/ml) reduced C. beticola symptoms by nearly 70% on distal, untreated leaves. Systemic resistance responses elicited by live B. mycoides Bac J and avirulent E. carotovora pv. betavasculorum isolates, measured by assays for chitinase and beta-glucanase, were statistically equivalent, and biphasic hydrogen peroxide production was observed. Although similar in timing, the second hydrogen peroxide burst was twofold lower for B. mycoides Bac J than for avirulent E. carotovora pv. betavasculorum. Hypersensitive cell death was elicited by avirulent E. carotovora pv. betavasculorum but not B. mycoides Bac J. An oxidative burst was elicited by spray-applied B. mycoides Bac J under both light and green light conditions, indicating that the signal produced by B. mycoides Bac J was not reliant on the stomata for entry into sugar beet. A working model for signal delivery and systemic resistance induction by B. mycoides Bac J in sugar beet is proposed.

Biological control of Pseudomonas syringae pv. syringae, the causal agent of basal kernel blight of barley, by antagonistic Pantoea agglomerans.

ABSTRACT Strains of Pantoea agglomerans (synanamorph Erwinia herbicola) suppressed the development of basal kernel blight of barley, caused by Pseudomonas syringae pv. syringae, when applied to heads prior to the Pseudomonas syringae pv. syringae infection window at the soft dough stage of kernel development. Field experiments in 1994 and 1995 revealed 45 to 74% kernel blight disease reduction, whereas glasshouse studies resulted in 50 to 100% disease control depending on the isolate used and barley cultivar screened. The efficacy of biocontrol strains was affected by time and rate of application. Percentage of kernels infected decreased significantly when P. agglomerans was applied before pathogen inoculation, but not when coinoculated. A single P. agglomerans application 3 days prior to the pathogen inoculation was sufficient to provide control since populations of about 10(7) CFU per kernel were established consistently, while Pseudomonas syringae pv. syringae populations dropped 100-fold to 2.0 x 10(4) CFU per kernel. An application to the flag leaf at EC 49 (before heading) also reduced kernel infection percentages significantly. Basal blight decreased with increasing concentrations (10(3) to 10(7) CFU/ml) of P. agglomerans, with 10(7) CFU/ml providing the best control. For long-term preservation and marketability, the survival of bacterial antagonists in several wettable powder formulations was tested. Over all formulations tested, the survival declined between 10- to >100-fold over a period of 1.5 years (r = -0.7; P = 0.000). Although not significant, storage of most formulations at 4 degrees C was better for viability (90 to 93% survival) than was storage at 22 degrees C (73 to 79%). However, long-term preservation had no adverse effect on biocontrol efficacy.

Distribution and Prevalence of Fusarium Crown Rot and Common Root Rot Pathogens of Wheat in Montana

Distribution of Fusarium crown rot (FCR) and common root rot (CRR) pathogens associated with wheat (Triticum aestivum) in 91 fields in Montana were determined during the 2008 and 2009 crop seasons using real-time quantitative polymerase chain reaction (qPCR) and conventional isolation methods. Correlations (P < 0.001) were found between detection methods for both diseases. FCR was detected in 57% of the fields and CRR was detected in 93% of the fields surveyed. Percent incidence based on isolation from individual tillers was Bipolaris sorokiniana (15%), F. culmorum (13%), and F. pseudograminearum (8%). FCR populations were highly variable across the regions and were not detected in any fields from the Gb5 soil types of Judith Basin and Fergus counties. The spatial distributions of FCR and CRR were affected by elevation, soil type, and temperature. High FCR populations were associated with spring wheat crops rather than winter wheat based on qPCR (P < 0.001). FCR and CRR could produce yield losses in a range of 3 to 35%. This study is the first time that qPCR was used to survey these two pathogen groups, and the merits and weakness of qPCR relative to traditional isolation methods are discussed.

A Coordinated Effort to Manage Soybean Rust in North America: A Success Story in Soybean Disease Monitoring

Existing crop monitoring programs determine the incidence and distribution of plant diseases and pathogens and assess the damage caused within a crop production region. These programs have traditionally used observed or predicted disease and pathogen data and environmental information to prescribe management practices that minimize crop loss. Monitoring programs are especially important for crops with broad geographic distribution or for diseases that can cause rapid and great economic losses. Successful monitoring programs have been developed for several plant diseases, including downy mildew of cucurbits, Fusarium head blight of wheat, potato late blight, and rusts of cereal crops. A recent example of a successful disease-monitoring program for an economically important crop is the soybean rust (SBR) monitoring effort within North America. SBR, caused by the fungus Phakopsora pachyrhizi, was first identified in the continental United States in November 2004. SBR causes moderate to severe yield losses globally. The fungus produces foliar lesions on soybean (Glycine max) and other legume hosts. P. pachyrhizi diverts nutrients from the host to its own growth and reproduction. The lesions also reduce photosynthetic area. Uredinia rupture the host epidermis and diminish stomatal regulation of transpiration to cause tissue desiccation and premature defoliation. Severe soybean yield losses can occur if plants defoliate during the mid-reproductive growth stages. The rapid response to the threat of SBR in North America resulted in an unprecedented amount of information dissemination and the development of a real-time, publicly available monitoring and prediction system known as the Soybean Rust-Pest Information Platform for Extension and Education (SBR-PIPE). The objectives of this article are (i) to highlight the successful response effort to SBR in North America, and (ii) to introduce researchers to the quantity and type of data generated by SBR-PIPE. Data from this system may now be used to answer questions about the biology, ecology, and epidemiology of an important pathogen and disease of soybean.

Temperature effects on the interactions of sugar beet with Fusarium yellows caused by Fusarium oxysporum f. sp. betae

Abstract Fusarium yellows, caused by Fusarium oxysporum f. sp. betae, causes significant yield and storage losses in sugar beet. The pathogen can be highly variable and sugar beet cultivars can have differing responses to fungal infection. Environmental factors may be contributing to this variability in disease response and these interactions have not been fully described. We describe how air temperature may influence interactions of F. oxysporum f. sp. betae with sugar beet. Fusarium yellows was assessed at different temperatures in five sugar beet cultivars to determine how increasing temperatures affect disease development. Generally, temperatures of 24°C or higher led to more disease symptoms, while temperatures of 16°C and lower led to fewer symptoms. Additionally, during resistant interactions, Fusarium yellows development did not increase as temperatures increased, indicating that resistance remained effective at higher temperatures. However, some F. oxysporum f. sp. betae isolates had varying abilities to cause disease at the different temperatures tested, with some isolates able to cause more disease at higher temperatures, while for other isolates, mid-range temperatures were optimum for disease development. To gain insight on disease development at higher temperatures, the growth rate of F. oxysporum isolates (both pathogenic and non-pathogenic to sugar beet) was assessed in vitro at 15°C to 35°C. The greatest growth tended to occur at 25°C; however, growth responses varied between isolates and did not always correlate with a disease response. These results suggest that diversity of the fungal population could influence disease severity in the field at different temperatures.

Enhancing the Efficacy of Bioherbicides

Plant pathogenic pseudomonads are inhibited by certain amino acids due to feedback inhibition or repression of key biosynthetic enzymes in amino acid biosynthesis pathways. As it turns out, plants are similarly inhibited by certain amino acids. These inhibitions can play a large role in plant pathology. For instance, Frenching disease of tobacco is caused by rhizosphere bacteria that overproduce the amino acid isoleucine. We found that overproduction of an inhibitory amino acid would greatly enhance the virulence of a plant pathogen greatly increasing its efficacy as a bioherbicide for control of noxious weeds.

INTEGRATING FUNGICIDE SEED, IN-FURROW FUNGICIDES AND FUNGICIDE BAND APPLICATIONS FOR IMPROVED CONTROL OF RHIZOCTONIA CROWN AND ROOT ROT

Barry J. Jacobsen, Ken Kephartand Alice Pilergam Department of Plant Sciences and Plant Pathology, Montana State University, Bozeman, MT. 59717-3150, 2. Montana Agricultural Experiment Station, Southern Agricultural Research Center, Huntley, MT. Introduction Rhizoctonia crown and root rot occurs in every region that sugarbeets are grown. The disease is caused by a complex of Rhizoctonia solani 2-2 isg (interspecific group) IV and isg III B, with the isg III B become more predominant in areas where corn is significant in rotations. Since the labeling of azoxystrobin (Quadris) in the mid 1990s growers have used banded and infurrow applications of this fungicide or products such as Headline, Gem or Proline. Efficacy of banded applications was greatly improved when researchers (Jacobsen, Khan) showed that applications must be made before soil temperatures at the 4 inch depth exceeded 70F and after soils at this depth were 60F, rather than a specific growth stage. However, timing for late planting or springs when soils warm up quickly was a problem or where growers often had a small window for optimal timing. In our 15 years of Rhizoctonia work in MT, the period for optimal timing ranged from 3-17 days depending on the spring. Growers with large acreages frequently could not get optimal timing on all their acres. While Rhizoctonia resistant varieties are available, they frequently have lower yield potential, do not have the full component of other disease resistances such as Rhizomania, Curly Top, Aphanomyces, Fusarium Yellows, and Cercsopora needed by our growers and in our experience have also responded to properly timed fungicide applications with both increased yields and reduced disease severity. This research was initiated to determine if a combination of seed treatments or infurrow fungicide application could be combined with banded fungicide applications such that growers would have a large window to apply banded fungicide applications and achieve a high level of control. Seed treatment fungicides will give some protection but alone do not provide satisfactory control. Infurrow fungicide applications have given inconsistent control with control being excellent in some years and poor in other years. In comparison, properly time band applications have always given the best control in our reseearch and that of most others. Also based on our storage work with Rhizoctonia infected beets that demonstrated that even a small percentage of infected beets in the storage pile will result in severe decay, both from Rhizoctonia and secondary bacteria and fungi), therefore improved control of Rhizoctonia crown and root rot is needed to manage storage losses. The Rhizoctonia problem in MT appears to be increasing in severity. I believe the primary reason is that our most popular varieties are more susceptible than varieties of 5-10 years ago. Also dramatic increase of corn in the rotation is suspected to contribute to higher inoculums levels in fields since it is a host to the isg IIIB strain. Based on work by Windels and her group we chose to closely examine penthiopyrad seed treatments as part of the package. Infurrow work done by the Michigan group indicated that infurrow applications of Quadris could also be part of the package.

Pathogenicity of Fusarium spp. to chickpea seed and seedlings (Cicer arietinum L.)

Abstract Damping-off disease can cause severe stand loss and reduce seedling vigour of kabuli chickpea (Cicer arietinum L.). While Pythium spp. are thought to be the primary cause of damping-off in chickpea, diversity and pathogenicity of other genera have not been widely explored. In surveys of three Montana fields of chickpea affected by damping-off, Fusarium spp. were isolated from 83% of the seeds tested, and were more common than the other taxa recovered, namely Pythium and Rhizopus. Fusarium spp. were identified to species using morphological characters and sequence analysis of two loci, the internal transcribed spacer (ITS) region of ribosomal DNA and the translation elongation factor (TEF) 1-α gene. Consistently identified species were F. oxysporum Schltdl. and F. redolens Wollenw. While discrepancies between identification methods made the identity of other isolates unclear, Fusaria infecting chickpea were diverse and varied across sites. The pathogenicity of eight isolates to kabuli chickpea was tested across a gradient of soil moisture and initial propagule density. All isolates were able to cause disease on kabuli chickpea seeds and seedlings. Effects of soil moisture levels on disease incidence varied among isolates, even isolates whose TEF 1-α sequences were 100% identical. This indicates that strains within a species can vary in pathogenicity. Additionally, most isolates caused more severe disease symptoms on cotyledons than roots.