CERCOSPORA LEAF SPOT CONTROL
VARIETY - FUNGICIDE - INTERACTION
1982

Rosemount Experiment Station
Plant Pathology Farm

Dr. H. L. Bissonnette
Professor, Extension Plant Pathologist

Twenty-five varieties of sugar beets were planted in a 2 acre plant disease nursery where each variety was replicated 3 times as single plots. The planting was subsequently divided, across the rows for the fungicide treatments.

The planting was inoculated with spores of the fungus Cercospora beticola, the causal agent of the Cercospora leaf spot disease. The inoculum contained both the natural and resistant strains of the fungus.

The experiment was set up to identify: control by fungicides; reaction to fungicide by varieties; and relationship of symptoms to disease loss. The varieties being grown appear to have various amounts of resistance to leaf spot. This resistance may respond to certain fungicides in such a way that a grower may have to select his fungicide according to the variety that he is growing. This phenomena might also express its affect in regard to the timing of the fungicide application or schedule.

To control a disease epidemic several steps must be followed -reduce the initial inoculum - reduce the build-up of the secondary inoculum - reduce the over-all population of the organism. This first experiment is only to look at the reaction of the varieties to fungicides, and does not attempt to solve each of the above concerns.

Only one application schedule was used in this experiment based on previous work. The schedule selected is one that most growers would use for a general control program. The schedule was four applications - with applications every 7 to 10 days, using amaneb-type funyicide (Dithane M-45), a tin hydroxide fungicide (Super Tin) and a copper fungicide (sit-Cop 4E). All chemicals were applied at label rates. The method of application in the case of the protectant fungicides is very critical as coverage and distribution of the fungicide on the leaf surface is necessary to prevent infection. As this was a disease garden -the treatments were not started until after the primary inoculation was established. I did not use a preventative spray schedule. The treatments were applied with a Bean High Pressure Sprayer (250 psi) in 40 and 60 gallons of water per acre. The first 2 applications were at 40 g/a and the last 2 at60 g/a. There are different types of spray applicators, each of which have different requirements. This experiment was not a test of methods of application.

The beets were thinned by hand to a 9 to 12 inch stand. All varieties were thinned the same, it may be that all varieties do not require the same thinning rate.

The yield data from the Rosemount Experiment Station does not reflect the yield expectations of commercial growers. However, the results obtained are relative to beets grown under disease conditions. The data from a statistical standpoint is significant at the 1% level of significance. Usually researchers are happy to have data significant at the 5% level in field experiments (appendix).

The plot was irrigated after inoculation (1 inch) early July. Due to rather dry conditions, a second irrigation was needed in early August. By late August and September water became a problem resulting in delaying harvest until mid-October. A fifth fungicide application may have been very advantageous, had I known that the harvest would be delayed almost 1 month.

The beets were lifted and hand topped. The sugar analysis was handled by the Southern Beet Cooperative through the cooperative efforts of Mr. E. Tanner and his laboratory crew.

The 25 varieties used were remnant seed (1981) from Dr. L. Smith. In this collection of seed obviously some varieties did not need to be tested, while some new varieties were not available. Ten varieties were selected by some of the sugar companies agriculturists, as being more significant to the growers.

RESULTS:
The varieties were rated on a symptom scale of 1 to 10 with 10 representing all leaves dead. The symptom ratings in Table 1 are an average rating of the 3 replications by variety and treatment, after treatments were completed. To have some idea of how a variety reacted to the disease pressure see the column identified as check in Table 1. Fungicide was not applied to the check. The symptoms ranged from a low 2.0 for ACH 14 to a high rating of 9.3 for MONO 833. All varieties when averaged together averaged 6.3. As might be expected there were differences in symptom expression. On Table 1A, listing the 10 selected varieties, it may be seen that the average of the 10 varieties (check) is 5.2, as compared to the 6.3 rating for all 25 varieties, indicating that these 10 varieties represent a higher degree of resistance, with the exception of variety 839 the highest rating is 6.7.

Looking at the various treatments (Table 1), it may be seen that each of the fungicide treatments resulted in slightly different degrees of symptom control. Across all varieties the tin hydroxides gave the best control of symptoms (Table 1).Comparing the 10 selected varieties, Table 1A, the differences between treatments is less. By comparing the data under the column (Treatment Variety Average) with the corresponding check one can see the general affect of fungicide treatment on each variety.

Yield and percent sugar data for the l0 selected varieties are listed in Tables 2 and 3 respectively. The yield as in Table 1, the variety response to the disease pressure is seen under the column check in Table 2. It is apparent that Cercospora Leaf Spot does reduce yield. By comparing the check column with the last column (variety treatment average) Table 2 it may be seen that all varieties did not respond to the same degree. Comparing data on Tables l and lA with the data on Table 2, it is seen that symptom expression is not necessarily reflected in yield. So what we have found is that there is a varietal by treatment interaction. This may be observed especially in the data for varieties Beta 1237 and ACH 14.

The average percent sugar data is found in Table 3. Purity and related data was not requested for this experiment. Again the check column shows the variety response to the disease pressure. It may be seen that the fungicide treatments do influence the % sugar; in all varieties the rnaneb fungicide resulted in higher % sugars.

CONCLUSIONS:
From a single years experiments, it appears that varieties respond differently to fungicide treatments, in symptom expression, yield and percent sugar, when grown in a disease nursery.

Cercospora Leaf Spot Disease crop loss can be reduced with a regular fungicide spray schedule. It may be possible to develop specific spray schedules and fungicide selection for specific varieties.

These results were obtained in a heavily infested disease nursery, however, the disease ratings were similar to natural occurring disease in commercial fields during the 1981 season.

SOME REASONS FOR CONCERN:
A disease epidemic occurs when a pathogen, in this example cercospora beticola, is in such high concentration in the beet crop field, that severe economic loss results from the infection process. The loss is the result of actual crop failure and the expense of chemical control.

Even with fungicide "control", the disease is abundantly present on certain varieties. This presence (abundant) is reflected in inoculum pressure or presence of a great amount of the fungus population.

As is often the case, we find that the very susceptible varieties are also the highest yielding varieties, considering the value of the crop, at first it appears that the growers best opportunity is to grow a susceptible high yielding variety, relying on the use of a fungicide to keep the loss at an economically acceptable level. Even though this is a successful operation - for the short run - it may well be devastating over the long term of growing sugar beets. The use of the currently available protectant fungicides does not eradicate the pathogen. The fungicide slows up the rate of infection which on susceptible varieties is excessive. In the case of susceptible varieties the amount of secondary inoculum produced is still excessive. So even though a grower may achieve an economic control of the crop loss effect, we end up with a large population of the fungus in the growing area.

It is within this over abundant population (an epidemic) of the fungus that new races or strains e.g. the systemic fungicide resistant strains, develop. The over dependence on the use of fungicides as the only disease control method, is or may besetting the stage for the possible development or resistant strains or races of the fungus that could again reduce the value of new fungicides.

The new fungicides, like the current systemic fungicides are very specific in their reaction to fungi, they are very effective in low dosage amounts. But it should be kept in mind that these very advantageous characteristics could lead to the loss of effectiveness if they must be continually used to control epidemics of a disease.

Growers will have to take additional measures to reduce the total amount of inoculum by growing the most resistant varieties, using fungicides for maximum control, utilizing sanitation practices, lengthening the rotation system and even may have to go to the extent of unconcentrating the growing areas. Controlling this disease will be expensive, as the saying goes, "there is no free lunch".

Some growers this past season found that they achieved, as good disease control with 2 applications of fungicide as their surrounding neighbors achieved with 4 or 5 applications. What they experienced was that their neighbors controlled to fungus population for them. If all growers elect to use a short spray schedule and continue to grow susceptible varieties who will control the population of this pathogen.


1982 Sugarbeet Research and Extension Reports. Volume 13, pages 227-235.


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