PROJECT RESULTS

Integrated Management of Leaf Spot Diseases in Wheat

Applicant: 

Dr. Lakhdar Lamari
Department of Plant Science
University of Manitoba
Winnipeg, Manitoba  R3T 2N2  Canada

Table of Contents:

• Background and Objective
• Procedure and Project Activities
• Results and Discussion
• Conclusions

ARDI Project:

#98-114

Project Status:

Completed November, 2000

Background and Objective:

The purpose of this study was to determine the effectiveness of several seed treatments supplied by Gustafson (3 years), Rhone-Poulenc (2 years) and Zeneca (1 year) in protecting wheat plants against leaf spotting diseases. The yield loss caused by leaf spotting diseases may be reduced if the amount of disease on the early leaf stages can be reduced.

Procedure and Project Activities:

Growth Room Experiments

In growth room experiments, the effectiveness of a set of chemicals on two pathogens was evaluated: Pyrenophora tritici-repentis (the cause of tan spot) and Septoria tritici (the cause of septoria leaf blotch). Both pathogens were evaluated on two wheat cultivars: Katepwa and Glenlea. Each experiment consisted of four pots of the six treatments. The plants were inoculated at either the 1^st, 2^nd or 3^rd leaf stage. Disease severity was assessed 7 and 20 days post-inoculation for tan spot and septoria leaf blotch, respectively. Throughout this study, a color image analysis system was used to quantify the amount of disease on the leaves. Each experiment was repeated at least twice.

Field Plot Experiments

Field plots were established in 1998 and 1999 at the Carman Research Station on land where wheat stubble remained from the previous year. The wheat stubble provided a natural source of inoculum. Samples of 20 leaves (one leaf per plant) from each plot were sampled at intervals during the growing season. This provided a means of determining the effectiveness of the treatments at different leaf stages. The leaf area affected was measured using an image analysis system. In 1999, a total of nine chemicals or mixtures of chemicals were tested.

Laboratory Experiments

Fungicides can have two effects on disease: they can reduce the amount of the leaf area affected, and/or they can reduce the number of spores produced. Because reducing the number of spores can lower the potential for spread, it can be just as important as reducing the amount of leaf area affected. Leaves were inoculated with the tan spot pathogen – Pyrenophora tritici-repentis. After allowing the disease to develop, infected leaves were placed in a high humidity chamber in order to favour the formation of spores. Once sporulation had occurred, the spores were washed off and counted.

Results and Discussion:

Growth Room Experiments

A complete set of results for first leaf infections in the growth room experiments is in the Appendix.  Based on results from the growth room experiments, two important conclusions could be reached:

1. The fungicides provided different levels of protection from leaf spotting diseases.

Figure 1 shows that Baytan at the 30gai/100 kg (grams active ingredient applied to 100 kg seed) reduced the amount of septoria leaf blotch by a factor of three over the untreated control. Baytan (15 gai/100kg) and Dividend also showed some effect, but this was limited to about a 40% reduction.

The effect of the seed treatments on tan spot is shown in Figure 2. Baytan (15 gai/100kg) reduced tan spot by a factor of three. The higher application rate of Baytan (30 gai/100kg) resulted in a four times reduction. Raxil and Dividend also had an effect, but significantly less than the Baytan treatments.

2. The protection provided by the best fungicides, in the context of these experiments, is measurable primarily at the 1^st and 2^nd leaf stages.
   
   Some residual protection was observed at the 3^rd leaf (mainly on septoria leaf blotch), although only a limited amount of testing was carried out at the 3^rd leaf and beyond.
   
   The results of growth room portion of this study suggest that seed treatments with some systemic fungicides may provide protection not only on their seed-borne targets, but may also be translocated to the leaves to provide additional benefit in reducing early damage by foliar pathogens. Because of the low costs associated with fungicide seed treatment, as opposed to foliar sprays, this area of applied research will be explored further in our research program to investigate the potential used of fungicide seed treatments in the early control of stubble-borne foliar pathogens.

Field Plot Experiments

The results for two of the treatments can be seen in Figure 3.

The results for all nine chemicals can be found in Table 1.

Table 1.  1999 Field Data

R-Raxil     B-Baytan     G-Gusai0001     M-Metalaxyl     I-Imidacloprid     V-Vitavax     T-Thiram

The ultimate purpose of reducing disease is to increase yield. Each plot was harvested and the yield weighed. Table 2 shows that there was no significant difference in yield between the treatments.

Table 2.  Effect of the treatment on yield. 1999 data.

R-Raxil     B-Baytan     G-Gusai0001     M-Metalaxyl     I-Imidacloprid     V-Vitavax     T-Thiram

Means with same letters do not differ significantly at P<=0.05.

One reason for the lack of significant difference in yield between the treatments could be inadequate disease pressure. If there is not enough disease pressure then no treatment, no matter how effective, will result in an increase in yield. To check for this, a subset of plots was treated with the foliar fungicide Tilt. Table 3 shows that plots treated with tilt had a significant yield increase. This means that there would have been enough disease pressure for a successful treatment to be detected. In addition to leaf spot diseases, rust was quite severe and may have contributed significantly to the reduction of yields shown in Table 3.

Table 3.  Effect of Tilt applications of yield showing that there was enough disease pressure to cause a disease loss.

Means with same letters do not differ significantly at P<=0.05.

Field results were not as conclusive as those obtained under a controlled environment. We observed some control at the 1^st leaf only, but subsequent measurements of disease on later leaf stages were not significant and, more importantly, there was no difference in yield. One of the problems identified in this type of evaluation is that of timing of infection. The weather was not conducive to infection at the time the 2^nd and 3^rd leaves emerged, resulting in low infection pressure even on the non-treated controls. As the season progressed, inoculum from non-treated plots and adjacent fields contributed to the development of the disease on our experimental plots, thereby canceling the basic hypothesis that early control of leaf spot diseases would result in lower inoculum levels. This confounding effect can only be circumvented by using large fields (> 100 acres) to ensure that most of the inoculum is of local origin. Only then could we test the effect of reducing inoculum at the 1^st - 3^rd leaves on late leaf stages and on yield. To prevent, or minimize the effect of rust, cultivars with known genetic resistance to this disease would have to be used.

Laboratory Experiments

Figure 4 shows that leaves produced from treated seeds produce considerably less spores per unit area. This suggests that the effect of fungicidal seed treatments may result in reduced sporulation.

Conclusions:

What have we achieved?

We have established that some systemic fungicide seed treatments can provide early control of foliar diseases such as tan spot and septoria leaf blotch, two major diseases of wheat in Manitoba.

We learned that small and medium size plots are not adequate for measuring the effect of seed treatments on foliar diseases, when the effects are only present early in the season (first few leaves)

What’s next?

In order to publish our results in a refereed scientific journal, we need to do more work to characterize the effect of the fungicides on disease development (sporulation of pathogen, fungal growth in tissue, residual protection at 3^rd leaf and after). We have also agreed with Gustafson to extend the study to spot blotch, a third leaf spotting disease. These studies are underway and will continue through the winter of 2001. We will submit a manuscript for publication shortly after.

Training

We have hired at least five undergraduate students over the two years of the project. These students were trained in basic plant pathology work at the field, lab and greenhouse levels.

Acknowledgements:

Funding for this project was provided by Gustafson, Rhône-Poulenc Canada Ltd., Zeneca and the Canada-Manitoba Agri-Food Research and Development Initiative.

Appendix:

Growth Room Data - First Leaf - The tables below provide a summary of the data for each experiment conducted on first leaves. Means with same letters in the grouping column do not differ significantly at P<=0.05. Each treatment consisted of four pots with six plants providing a total of 24 leaves. Treatment 1 is the untreated control. Glenlea is resistant to Septoria so this combination was not tested.

Pyrenophora on Glenlea 1^st leaves - Experiment 1

Pyrenophora on Glenlea 1^st leaves - Experiment 2

Pyrenophora on Glenlea 1^st leaves - Experiment 3

Pyrenophora on Katepwa 1^st leaves - Experiment 1

Pyrenophora on Katepwa 1^st leaves - Experiment 2

Pyrenophora on Katepwa 1^st leaves - Experiment 3

Septoria on Katepwa 1^st leaves - Experiment 1

Septoria on Katepwa 1^st leaves - Experiment 2

Septoria on Katepwa 1^st leaves - Experiment 3

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