SUNFLOWER CROP SURVEY : DISEASE ASSESSMENT ACROSS EIGHT STATES THOMAS J. GULYA

Transcription

1 SUNFLOWER CROP SURVEY : DISEASE ASSESSMENT ACROSS EIGHT STATES THOMAS J. GULYA Research Plant Pathologist USDA-Agricultural Research Service, Northern Crop Science Laboratory Fargo, ND gulyat@fargo.ars.usda.gov Summary The annual NSA-coordinated fall yield survey was enlarged in 2001 to include enumeration of weeds, insects and diseases in the northern Plains sunflower production area (North Dakota, South Dakota), and in 2002 the survey was expanded to include Minnesota and five states of the central High Plains area (Colorado, Kansas, Missouri, Nebraska, and Texas). A total of 477 fields were surveyed in 2002 in eight states by 46 teams. Survey participants received training in insect, weed and disease identification. Diseases enumerated included three Sclerotinia-incited diseases: head rot, wilt (basal stalk rot) and mid-stalk rot, Phomopsis stem canker, Rhizopus head rot, rust (Puccinia helianthi), downy mildew, white rust (Albugo), charcoal rot and Verticillium wilt. Averaged across all eight states, the most prevalent diseases were Verticillium wilt, Rhizopus head rot, and Sclerotinia head rot, with 5.4%, 4.3%, and 3.3% of the crop affected by each disease, respectively. In North Dakota (ND), which has 54% of the 2002 U.S. acreage, the most prevalent diseases were Sclerotinia head rot (4.7%), Rhizopus head rot (2.8%), and Sclerotinia wilt (2.5%). One significant change from 2001 to 2002 was the dramatic increase in Verticillium wilt incidence in South Dakota (SD) from 2% in 2001 to 18% in Over all the entire U.S. sunflower production area, however, diseases were considered a minor production constraint. Surveyors did not cite diseases as the number one yield-limiting factor in any fields in six of the eight states. In ND, diseases were cited as the leading yield limiting factor in only 18% of the 265 inspected fields. Continued collection of disease data in each annual survey will help establish a baseline of disease incidence by region, and will be helpful to researchers in directing efforts towards the most important diseases. Materials and Methods Forty-six teams of surveyors inspected 477 fields in eight states from the middle of August (Texas) until the third week of September (North Dakota) when sunflower fields were at physiological maturity. This sunflower growth stage was chosen since yield was the main criteria of interest, acknowledging that many foliar diseases would be underestimated because senescence or frost may have damaged leaves and obscured foliar disease symptoms. Following procedures established for previous years yield surveys, approximately one field for every 5000 acres of 2001 sunflower production was inspected per county. Participants in the survey received a one-day training session in

2 insect, weed and disease identification and followed standardized procedures in each state. A sheet of color photographs depicting typical symptoms of the harder to identify diseases (Figure 1) was given to each team of four participants, and one person was designated to collect disease data in all fields to insure continuity. The protocol was to inspect three sites per field, starting fifty paces into the field. At each of the three sites, a strip of 17 contiguous plants were counted off, and then the number of plants within that strip inspected for the occurrence of each of the ten diseases being monitored. Those diseases included (1) Sclerotinia head rot, (2) Sclerotinia basal stalk rot or wilt, (3) Sclerotinia mid-stalk rot, (4) Phomopsis stem canker, (5) Rhizopus head rot, (6) downy mildew, (7) rust (caused by Puccinia helianthi), (8) charcoal rot, (9) white rust (caused by Albugo tragopogonis) and (10) Vertcillium leaf mottle or wilt. The number of plants showing symptoms of a disease was recorded (regardless of the severity). The one exception was rust. Since rust, if present, was assumed to be of uniform distribution within a field, foliar severity was recorded rather than incidence. The procedure was to estimate the approximate percentage of leaf pustule coverage on the uppermost four leaves, using a pictorial guide as depicted in Figure 2. Upon inspecting the three sites of 17 plants each (=51 total plants), the surveyor then calculated the percent incidence for each disease, which, for the sake of simplicity, was twice the number of diseased plants. The team leader entered the data into an Excel spreadsheet and the data was ed to a central collection site and ultimately put into a master spreadsheet, which is accessible on the internet at website also has maps of each state showing the sites of surveyed fields and shading to depict the incidence of each disease. In addition to collecting disease incidence data, each team was asked to collect two soil samples. One sample, bulked from three sites within each field and totaling about a cup or 500 cc, was for analysis of the presence of Sclerotinia mycoparasites. Secondly, each team was asked to collect one larger sample (one gallon ~ 4 liters, bulked from three sites within a field) from every third field. This large sample was to be used in a greenhouse seedling bioassay to detect the presence of the downy mildew fungus, and thus compare greenhouse test results with actual field disease observations. Results and Discussion The number of fields inspected per state is shown in Table 1. While the objective was to proportionately inspect one field per 5000 acres of production, this ratio was not quite met in some areas of the central High Plains states. For a complete analysis of the agronomic and yield portion of the NSA survey, please see the paper by Lamey and Dietrich on this same website.

3 Table 1. Summary of 2002 sunflower field survey statistics by state. State 2001 acres (x 1000) 2002 acres (x 1000) Est Yield (#/acre) # fields inspected % fields oilseed % fields with disease rated as #1 problem North Dakota South Dakota Minnesota Nebraska Colorado Kansas Missouri Texas U.S. Total The major sunflower diseases observed in the fall survey across the eight-state region were Verticillium wilt, Rhizopus head rot and Sclerotinia head rot, with 5.4%, 4.3% and 3.3.% incidence, respectively (Table 2). Verticillium wilt was only recorded in two states (ND and SD), but the disease had an extremely high incidence in SD (18%). Collectively, the three diseases caused by Sclerotinia sclerotiorum (head rot, wilt and mid-stalk rot) were as prevalent as Verticillium wilt, with an incidence of 5.4%. Across the eight-state region, Rhizopus head rot has become a significant disease. Rhizopus head rot was the most prevalent disease in four states and the second-most prevalent disease in three other states. White rust was not observed in any field across the entire eight-state region. Downy mildew incidence was quite low at 0.2% incidence, but this was expected as this seedling disease is consistently underestimated by fall surveys. State by state there was considerable variation in the prevalence of individual diseases. For example, Rhizopus head rot was generally more severe in the Central Great Plains and decreased in severity in the more northern states. To a large extent this was expected since Rhizopus head rot is generally more common in areas having high summer temperatures and low rainfall, and is also known to be associated with insect damage, such as that caused by the sunflower head moth (Homoeosoma electellum) (Rogers et al., 1979; Klisiewicz, 1978). Sclerotinia incidence for all three disease phases was greatest in the Dakotas and Minnesota and totally absent in the Central Great Plains. This stark contrast in disease occurrence is most likely related to the long history of Sclerotiniasusceptible crops in rotations in the Dakotas coupled with a three decade long history of sunflower production. In contrast, KS, CO, NE and TX have a relatively short history of sunflower production coupled with rotations mainly involving non-hosts of Sclerotinia such as wheat, corn, sorghum and cotton. Examining disease incidence on an individual state basis, MN had the highest incidence of Sclerotinia head rot (7.3%) and nearly as much Phomopsis stem canker (3.7%) as did Colorado. Although the incidences of most diseases in MN were higher than in adjacent

4 ND and SD, the relatively small acreage in MN diminishes its impact upon total U.S. sunflower statistics. North Dakota and South Dakota had the greatest diversity of diseases, with nine out of ten monitored diseases recorded in both states. In contrast, the states in the central Great Plains had only two to four diseases recorded. Table 2. Sunflower disease incidence measured by the fall 2002 NSA survey. Disease Disease Incidence (% crop affected) by state USA ND SD MN NE CO KS MO TX Sclerotinia Stalk Rot Sclerotinia Head Rot Sclerotinia Mid-Stalk Rot All Sclerotinia diseases Phomopsis Stem Canker Rhizopus Head Rot Downy Mildew Rust (Puccinia) White Rust Charcoal Rot Verticillium Wilt Some of the data recorded in the survey needs to be viewed cautiously and substantiated by subsequent surveys and/or further studies. For example, the high incidence of Phomopsis stem rot and Rhizopus head rot in Missouri is a statistical artifact since only one field was inspected in that state. The incidence of Verticillium wilt in SD was also very puzzling. In the 2001 survey, Verticillium wilt incidence in SD was 2% compared to 18% in the 2002 survey. Upon inspection of the 2002 SD data, one can see that Verticillium incidence was highest in the south central portion of the state (Figure 3). Verticillium wilt was recorded in 24 of 27 inspected fields in the south central portion of SD in 2002, with this region having an average incidence per field of 59 %. Sixteen of the 27 fields (60%) had greater than 50% Verticillium incidence, and 11 fields (40%) had > 90% Verticillium incidence. Since the rest of SD regions had Verticillium incidences ranging from 2 to 18%, one wonders what factors may have contributed to the high incidence in the south central region. Possible explanations include (1) a preponderance of Verticillium-susceptible hybrids in this region, or (2) the occurrence of a new Verticillium biotype, able to overcome the existing resistant hybrids, within this region. Four different teams surveyed this region, and all teams recorded fields with high Verticillium incidence, so observer bias or misidentification was not likely. Comparison of 2002 disease incidence with 2001 is only possible with data from ND and SD because the NSA survey was only expanded to include eight states this year. In general, the 2002 growing season was more conducive for sunflower diseases than the 2001 season. Sclerotinia head and stalk rot incidences were higher in both states in 2002, as was that of Rhizopus head rot. The incidence of Phomopsis stem canker decreased in

5 ND in 2002 compared to 2001, while in SD the reverse trend was observed. The greatest difference between the two years was the amount of Verticillium observed in 2002, mentioned previously. No similar change in Vericillium incidence was noted in ND fields. Figure 3. Vertcillium wilt severity in South Dakota sunflower fields surveyed in Table 3. Changes in sunflower disease incidence in North Dakota and South Dakota from 2001 to Disease North Dakota South Dakota Sclerotinia Wilt 2% Sclerotinia Head Rot Sclerotinia Midstalk Rot All Sclerotinia Phomopsis Stem Canker Rhizopus Head Rot Downy Mildew NR* 0.3 NR 0.05 Rust (Puccinia) Charcoal Rot Verticillium Wilt *NR= Downy mildew not recorded in 2001 NSA survey.

6 One benefit of having data collected on multiple traits is the ability to compare the impact of one or more traits (agronomic, cultural, insect, weeds) on disease incidence. Table 4 presents T-test comparisons of disease incidence broken down by row spacing, hybrid type, and tillage categories. One question that producers and researchers have long asked is whether one tillage practice is better than another with regard to the survival of Sclerotinia and disease in a subsequent crop. Based on the information gathered in 2002, it appears that tillage produced no significant differences in the incidence of any Sclerotinia disease on sunflower. However, there was a significant difference in the incidence of Phomopsis stem canker, Rhizopus head rot and downy mildew with regard to tillage practices. For all three diseases, the least amount of disease was observed under no-till conditions and the highest disease incidence was in fields with conventional tillage. Minimum tillage produced an intermediate amount of disease between no-till and conventional tillage. Previous studies on the effect of tillage on disease incidence have usually shown that conventional tillage, by burying infected crop residues, generally results in a decrease in disease incidence. There is a possibility that tillage practices may be confounded within other factors (i.e. geographic region) that also impact disease incidence in this survey. Analysis of the impact of tillage practices over several years should, however, produce some trends that have sound biological explanations and can be used by growers to minimize disease. Table 4. The impact of hybrid type, row width, and tillage practices on the incidence of selected sunflower diseases across eight states in the 2002 NSA survey. Highlighted cells are significantly different at the 5% level according to a T-test. Verticillium Wilt Charcoal Rot Downy Mildew Rhizopus Head Rot Phomopsis Stem Canker Sclerotinia Midstalk Sclerotinia Head Rot Sclerotinia Wilt Number of fields <22 row width > 22 row width Oil Hybrid Confection Hybrid No Tillage Minimum Tillage Conventional Tillage The influence of hybrid type and cultural practices on disease incidence is presented in Table 4. Row width had no appreciable effect on the incidence of any disease, except

7 Verticillium wilt, where sunflower planted in conventional row widths (i.e. > 22 ) had more disease. While statistically significant, this phenomenom may be due to the fact that Verticillium wilt was very severe in south central SD where all 27 fields had conventional row widths. Most seed companies will acknowledge that confection hybrids are generally less resistant to most diseases compared to oil hybrids. However the data from the NSA survey, based on 78 confection fields and 394 fields with oil hybrids suggested that this generalization may not always be true. Confection hybrids statistically had more Rhizopus head rot than oil hybrids, but the reverse was true for Sclerotinia head rot. As mentioned previously with tillage practices, however, further analysis will need to be done to statistically remove the possible confounding influence of geography and thus environmental influences. If geographically paired confection and oilseed fields are compared, the statistical analysis will have a more precise biological explanation. Surveys such as this often tend to dwell on the presence of factors while overlooking the significance of negative occurrences. It should be pointed out that a majority of sunflower fields either had no diseases or the diseases present were not considered the major yield limiting factor (Table 1). In six out of eight states, diseases were not considered the number one limiting factor. This relative absence of disease in most fields is also shown when examining data from North and South Dakota, the two states having the greatest sunflower production area (Table 5). When examining most individual diseases, the majority of fields in both states had zero incidences. If one takes a 5% disease incidence as a point above which yield losses are likely to occur, then one can enumerate what proportion of fields may have been negatively impacted by various diseases. In North Dakota, for example, 29% of the fields had >5% incidence of Sclerotinia head rot and 15% of the fields had >5% incidence of Sclerotinia stalkrot, thus showing that Sclerotinia continues to be the major disease causing losses in ND. In contrast to ND, Rhizopus head rot and Verticillium wilt were the two diseases in SD likely causing the most yield losses. Table 5.. Percentages of sunflower fields in North Dakota and South Dakota in 2002 with varying incidences of the major diseases. North Dakota South Dakota Disease % Fields with Incidence Maximum Incidence % Fields with Incidence Maximum Incidence 0% >5% >10% 0% >5% >10% Sclerotinia Head Rot Sclerotinia Stalk Rot Phomopsis Stem Canker Rhizopus Head Rot Verticillium Wilt The soil samples collected during the NSA survey are being processed during the 2002/3 winter and the results will be presented at the next Research Workshop. Soil samples intended for the bioassay to detect sunflower downy mildew need to be stored for a

8 period of at least four months to allow the mildew spores time to mature and become infective. The analysis of soil samples for identification of Sclerotinia mycoparasites is partially finished. The major fungi being recovered are Trichoderma spp., Gliocladium spp., Coniothyrium minitans and Sporidesmium sclerotivorum. The latter two fungi are documented mycoparasites of Sclerotinia, and C. minitans is the active component of the commercial biocontrol product Intercept. If C. minitans is relatively scarce, it would provide justification for using Intercept, while, conversely, if C. minitans is widespread, it would suggest that Intercept may be of questionable benefit for Sclerotinia control. Isolates of C. minitans will also be categorized by the ph of the soil from which they were collected. Since the majority of ND and SD soils are ph > 7, and this fungus generally prefers soils < ph 7, finding a C. minitans isolate that had a higher ph optimum than the isolate in Intercept may be useful to develop a biocontrol agent more adapted to survival under soil and weather conditions in the northern Great Plains. Conclusions The inclusion of information on the prevalence and indentification of weeds, insects and diseases in the annual NSA-sponsored survey has greatly increased the value of this effort. The survey is labor intensive, but with multiple teams conducting the surveys in their respective states, the survey can be accomplished in a relatively short time period. Adequate training of all participants prior to the survey is essential if the data is to be accurate however, so efforts need to be made every year to insure that all individuals are competent in the area they are responsible for. Tabulation of survey results over the years will establish a baseline and timeline for the occurrence of various pests, and should help researchers decide what the most important topics to study are, and which weeds, diseases and insects are increasing, and thus need to be addressed before they become serious problems. The survey data should also help to understand the interaction between various problems (weeds, insects, diseases) and climate, agronomic and cultural factors. By having the data available on a website, this information will be of great benefit to producers, seed companies, and public and private researchers. Literature Cited Klisiewicz, J. M Relation of infestation with sunflower moth (Homoeosoma electellum) larva to the incidence of Rhizopus rot in sunflower seed heads. Can. J. Plant Sci. 59: Rogers, C. E., T. E. Thompson, and D. E. Zimmer Rhizopus head rot of sunflower: etiology and severity in the southern Plains. Plant Dis. Rept. 62:

9 Figure 1. Color photo sheet used to train participants in the in annual NSA survey.

10 Figure 2. Computer generated diagrams of three different levels of rust pustule coverage on a sunflower leaf. Surveyors are asked to estimate the severity of rust on the upper - four leaves and pick one of these three categories.