Screening fungicides for control of Phytophthora root rot of caneberries (raspberry)

Project Code SCR07-004

Project Lead

Janice Elmhirst - Elmhirst Diagnostics and Research 

Objective

Screen new fungicides with demonstrated activity against Oomycete diseases for potential control of Phytophthora root and crown rot of raspberry and crop tolerance

Summary of Results

Root and crown rot caused by Phytophthora fragariae var. rubi causes serious crop loss each year to raspberry growers in British Columbia, Ontario, Quebec and the Maritimes.

Of the two main raspberry cultivars grown in British Columbia, cultivar 'Malahat' is the most prone to disease and cultivar "Meeker", which comprises the vast majority of the processing crop, has become increasingly susceptible to disease in recent years, resulting in losses of 30-50% in many fields.

RIDOMIL GOLD 480 EC (metalaxyl-m) has been used for more than 10 years to control Phytophthora rubi, but in recent years, growers have not obtained good control of raspberry root rot with this product, probably due to the development of pathogen resistance to metalaxyl or increasing degradation of the product by soil microorganisms.

This experiment was therefore conducted with the objective of finding new reduced-risk fungicides to control Phytophthora in raspberries.

The experiment was conducted in Abbotsford, British Columbia. Raspberry plants (cultivar Meeker) from in vitro culture were transplanted into one gallon pots on August 10, 2007. The pathogen used to inoculate the raspberry plants was isolate AAFC #16184 of Phytophthora fragariae var. rubi. The plants were inoculated with Phytophthora rubi on September 14 and October 4, 2007.

The experiment was a randomized complete block design with four replicates per treatment. The treatments evaluated included 14 different fungicides as well as a non-inoculated control and a control inoculated with the fungus. The fungicide drenches were applied 48 hours prior to inoculation with the pathogen. Two fungicide treatments were applied by foliar spray 14 and 28 days later compared to the treatment by drench application. The products were applied once, at the highest rate recommended by the manufacturer.

The raspberry plants were evaluated for phytotoxicity 24, 48 and 72 hours after each fungicide application and again at one and two weeks. At the end of October, the plants were visually examined for foliar symptoms of root rot. Another visual examination of overall quality (growth, vigour, colour) was performed in mid-November, when the raspberry plants were harvested. At this time, the plants were removed from the pots and the roots were removed, washed, dried and weighed. The percent root rot was estimated and whole plant fresh weight was measured.

The most consistent quantitative factors measured within each treatment and the most reliable indicator of disease severity was root fresh weight and dry weight. The data obtained on the visual examination of plant quality were quite variable. This result could be due to the fact that the plantlets from in vitro culture were not all the same size at transplanting, which probably affected the final plant size. No foliar phytotoxic symptoms were observed in any of the treatments.

Crown rot could not be evaluated during this trial since root crowns do not form in raspberry plants in their first year of development following in vitro culture.

Pre-stop (Gliocladium catenulatum) was the fungicide that gave the best results in terms of root weight, percent root rot, whole plant weight and overall plant quality. Four other fungicides - Tanos (famoxadone + cymoxanil), Pristine (boscalid + pyraclostrobin), Compass (trifloxystrobin) and Curzate (cymoxanil) - yielded satisfactory results. These reduced-risk pesticides should be tested again in the future.

This trial was conducted from September to November and the cool temperatures, while favourable for P. rubi root infection, may have slowed disease progress and plant growth. In a previous study conducted from April to June under warmer conditions, Pristine (boscalid + pyraclostrobin) yielded the best results. The fungicides Allegro (fluazinam) and Ranman (cyazofamid) also performed well, reducing the level of root and crown rot.

In the future, it would be interesting to conduct other trials with the fungicides Allegro (fluazinam) and Ranman (cyazofamid) since temperature appears to affect their efficacy. Also, prevention of crown infection may have a greater impact on raspberry plants in the field than suppression of root rot alone. It would therefore be useful to conduct additional trials with raspberry plants that have root crowns.

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