PROJECT RESULTS

Control of Late Blight and Secondary Soft Rot in Storage

Applicant: 

Gary Sloik
Keystone Vegetable Producers Association
Portage la Prairie, Manitoba  R1N 3K5  Canada

Table of Contents:

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

ARDI Project:

#99-283

Project Status:

Completed January, 2001

Background and Objective:

There were over 75,000 acres of potatoes produced in Manitoba in 2000. The most serious diseases affecting this crop in Manitoba are late blight and secondary soft rot. Based on monitoring by Manitoba Agriculture and Food, losses from late blight and secondary storage rot over the past five years have been close to fifteen per cent. Individual losses have been as high as ninety per cent. The cost of this yield loss, over two million dollars, could be reduced by storage application of chlorine dioxide.

The objective of this study is to determine the effectiveness of chlorine dioxide for reducing the tuber to tuber spread of late blight and secondary soft rot in stored potatoes. This information is required to confirm whether this product is effective in reducing storage losses. This product will be evaluated under commercial storage conditions and under laboratory conditions.

Procedure and Project Activities:

The ability of chlorine dioxide to reduce soft rotting of potatoes was tested using the method of Lund and Wyatt (1979). They accelerated the onset of soft rot by placing potato tubers into a hypoxic environment created by putting the tubers into a closed container and replacing the air with nitrogen gas. This method was preferred over an alternative method that keeps the tubers wet for extended periods of time, which may result in dilution of the chemical treatment.

Laboratory Applications

Tests were conducted on two similar commercial products: Purogene made by Biocide, and Anthium which is made by International Dioxide. When acidified, both products release the gas chlorine dioxide, which has been reported to protect potatoes by killing bacteria on the tuber surface. For each chemical, two experiments were conducted using the variety Shepody and one experiment was conducted using the variety Russet Burbank for a total of six laboratory application experiments.

Each experiment consisted of four treatments of 0, 50, 200 and 400 ppm of chlorine dioxide (with the exception of one Shepody experiment with Purogene where three treatments of 0, 50 and 400 were used) and each treatment consisted of five tubers.

The tubers were inoculated with Erwinia carotovora pv atroseptica by dipping them into a bacterial suspension containing approximately 10⁷ bacterial per ml. After inoculation, the tubers were allowed to dry before being sprayed with the appropriate treatment.

The products were activated according to the manufacturer’s directions and applied to potatoes using a Dynafog applicator. The Dynafog applicator produces a fine mist and is the same instrument used by commercial applicators. The chemical solutions were applied to the tubers at the concentrations under study until runoff was apparent. For the 0 ppm level, distilled water was applied to one set of tubers to provide an untreated control.

Storage Applications

In these experiments, Purogene was applied using a commercial applicator. Storage application tests were conducted with Purogene alone. Anthium could not be tested in commercial storages because it is not licensed for use in Canada.

Approximately 50 pounds of tubers were removed from the storage one to two days before the storage was to be treated. The tubers were taken back to the laboratory, washed and inoculated with E. carotovora pv atroseptica. The tubers were divided into twelve lots of five tubers and each lot was placed into a plastic mesh bag. The next day, six of the bags were returned to the storage and buried in the top of the pile at different locations. The other six bags remained at the laboratory and were kept at 8^oC. Approximately 24 hours after the storage was treated, the six bags of tubers were removed and returned to the lab. All twelve bags were then placed into a low oxygen chamber and treated in the same way as tubers receiving chlorine dioxide treatments in the laboratory.

Disease Assessment

Ten days after being placed into the low O₂ containers, the tubers from both the laboratory and storage application studies were assessed for soft rot. Each tuber was weighed before and after soft rotted tissue was removed. The difference in weight was compared to the original weight of the tuber to provide a percentage of rot.

The effect of Purogene on late blight infection was also tested. For each experiment, ten Russet Burbank tubers were inoculated by placing a drop containing 500 Phytophthora infestans zoospores onto three eyes of each tuber. Immediately after inoculation, 400 ppm Purogene was applied using the Dynafog applicator until runoff was observed. Seven days after inoculation, the inoculated eyes were cut through and the presence or absence of disease was noted. The number of infected eyes was used as a measure of the effectiveness of the treatment.

Results and Discussion:

Laboratory Application – Soft Rot Control

The amount of rot that developed on tubers exposed to various concentrations of chlorine dioxide was quantified using two different measures. The average rot per tuber was calculated by weighing each tuber before and after the rot was removed. The difference in weights was used to calculate the percentage of the tuber that had rotted. This data is shown in Tables 1 and 2. The experiments in Table 1 used Purogene. The experiments in Table 2 used Anthium. The other measure used was the percentage of tubers in a sample that had evidence of rot. This data is shown in Tables 3 and 4.

Although it is possible to detect differences in the amount of rot between treated and untreated tubers in various experiments, the value of these comparisons is limited. First, the large standard deviations within each experiment make it difficult to calculate statistically significant comparisons. Within an experiment, the rot on some tubers was barely detectable, while other tubers were almost completely rotted away. The large differences were probably the result of differences in the ability of individual tubers to resist the progression of the rot.

Secondly, the amount of rot in a tuber is of limited importance when considering disease development in a commercial storage. The amount of disease on a tuber is dependent on the time that the measurement was taken. Once a tuber is infected with soft rot, the lesion will continue to grow until the tuber is completely consumed. In a storage situation, once a tuber is infected with soft rot, it will almost certainly spread the disease to its neighbours. Therefore, it is the presence of soft rot on a tuber that is important, not the degree of soft rot.

Table 1. Effect of Purogene on the amount of rot found on tubers.

Table 2. Effect of Anthium on the amount of rot found on tubers.

Table 3. Effect of Purogene on the number of tubers that developed soft rot.

Table 4. Effect of Anthium on the number of tubers that developed soft rot.

In the first Shepody experiment, when Purogene was applied at 400 ppm, the protection appears to be minimal. When applied at the same concentration to Russet Burbank potatoes, protection also appears to be limited. However, in the second Shepody experiment, there is a threefold reduction in the percentage of infected potato tubers.

Similar results were seen when 400 ppm Anthium was applied (Table 4) except in this case; two out of the three experiments showed a large decrease in the number of infected tubers when Anthium was applied. In the first Shepody experiment, the difference between untreated and treated tubers is minimal. However, in the Russet Burbank and second Shepody experiment, there are large differences in the number of infected tubers between the untreated tubers and tubers treated with 400 ppm Anthium.

Storage Applications

The ability of Purogene to protect potato tubers was also tested in a commercial situation. The results from the four bins sampled can be seen in Table 5. The results were similar to those seen in the laboratory application in that there was considerable variation between experiments. In (Farm 2 Bin 1) the reduction in the probability of infection was reduced three-fold. However, in the other bins the reduction was less than one-fold.

Late Blight Control

Purogene and Anthium were applied to tubers immediately after several eyes had been inoculated with P. infestans zoospores. Two experiments were conducted with each product. The results for the two products are shown in Tables 6 and 7. As with the soft rot tests, there again appears to be considerable variation between the experiments. If Experiment 1 with Purogene is ignored, the amount of control provided by both products looks good. However, when Experiment 1 of the Purogene tests is taken into account, there appears to be some variable in either the application of the chemical or the health of the tuber that caused the degree of control to be reduced.

Table 5. Effect of Purogene treatment applied to a commercial storage on the number of tubers that developed soft rot.

Table 6. Effect of Purogene on late blight infection.

Table 7. Effect of Anthium on late blight infection.

Conclusions:

Chlorine dioxide was observed to reduce the amount of soft rot in potatoes, but the effect was not consistent between experiments. The variability of results was seen both when chlorine dioxide was applied in the laboratory and when it was applied under commercial conditions. The lack of consistency could be due to many factors, but a likely explanation is changes in the health of the tubers. A potato tuber that is healthy and, therefore, has some natural resistance to soft rot, may respond better to the chlorine dioxide than an unhealthy tuber that has little natural resistance. Chlorine dioxide will probably only increase the storage life of a potato crop if all the other management strategies that go into producing a healthy crop are adhered to. Therefore, when considering the use of this chemical, it is important for growers to consider it as only part of a storage disease control program.

Differences in the amount of soft rot were seen when chlorine dioxide was applied to potatoes in both laboratory and commercial settings. Some protection against late blight was also noted. The results suggest that chlorine dioxide does offer some protection against soft rot and late blight and this may delay the spread of the disease through a storage. Predicting the length of the delay will be difficult because of the size and complexity of commercial storages. The length of the delay will have to be estimated by the producers who will need to combine their own experience with information from studies like this, to determine whether or not application of chlorine dioxide is economically viable.

Acknowledgements:

Other project partners supporting this project include Bio-Cide International Inc., International Dioxide and Stanchem Inc.

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