Screening of fungicides for needlecast diseases on conifers

Project Code SCR08-050

Project Lead

Jim Jotcham  Marbicon Incorporated

Objective

To assess the potential of selected fungicides for needlecast control on two conifer species, Colorado spruce (Picea pungens) and balsam fir (Abies balsamea) and to provide recommendations for candidate fungicides to be pursued for registration in Canada. In addition, methods to detect the presence of the pathogens in asymptomatic needles will be developed.

Summary of Results

Background

In Canada, needlecast diseases present some of the greatest challenges for commercial conifer tree growers and are also a significant issue on conifers that are established in the landscape and in windbreaks. Two common needlecast diseases include Lirula needlecast caused by Lirula nervata on balsam fir and Rhizosphaera needlecast caused by Rhizosphaera kalkhoffi on spruce. The presence of Lirula needlecast can result in conifer tree growers having to trim off the lower diseased branches in order to have saleable Christmas trees, thus significantly lowering the wholesale value of the trees. The presence of Rhizosphaera needlecast on spruce is a concern in Ontario because of the unsightly appearance of infected foliage and threat it poses to tree health. At the initiation of this project, recommendations for the control of needlecast diseases were to apply fungicides to the current season's growth from bud break to hardening off (late summer). The few fungicides available to control needlecast diseases often did not provide adequate control. Screening trials and subsequent efficacy trials that lead to new fungicide registrations for conifer needlecasts, would arm growers and landscapers with efficacious rotational products.

Approaches

In this multi-faceted trial, seven new active ingredients, selected based on their efficacy against related fungi and availability, were screened for efficacy against needlecast diseases.

Experiments were conducted in 2008 and 2009 at two sites in Ontario and Nova Scotia. At the Ontario site, the main target pests were R. kalkhoffi and Stigmina lautii on blue spruce; at the Nova Scotia site, the main target pest was L. nervata on balsam fir. A number of fungicide active ingredients including azoxystrobin, myclobutanil, propiconazole, pyraclostrobin and boscalid, trifloxystrobin, sulphur and mancozeb + myclobutanil (Nova Scotia only) compared to the industry standard, chlorothalonil, were evaluated at both sites. In 2008, three spray applications of fungicide were made to each test branch, with a target interval of 10 days, starting at shoot emergence.

Spore production of needlecast fungi was assessed at both sites, through weekly collections of twigs from at least three trees per location. From these twigs, needles of various ages were examined in a laboratory at the University of Guelph for the presence of symptoms of disease and for signs of pathogens. To determine the exact pathogens present, the pathogens were isolated and DNA sequenced.

As symptom development for needlecast disesases can take as long as two years, twig and needle samples were also sent to the Ontario Forest Research Institute for development of diagnostic procedures to assist in quicker disease identification. For those pathogens for which molecular detection methods were developed, attempts were made to quantify amounts of pathogen present in fungicide treated foliage. 

Results

At the Nova Scotia site, symptoms caused by L. nervata took a year to develop on balsam fir. Sporulation of Lirula (ascospores) occurred mostly during the month of June, during the fungicide treatment period. Differences among treatments were apparent in the fall 2009 evaluation of the 2008 foliage however none of the treatments showed significant efficacy. An attempt to develop a DNA probe for the earlier detection of Lirula spp. was not successful.

At the Ontario site, sporulation by S. lautii occurred all summer, and sporulation by R. kalkhoffii peaked in the 2007 needles in October 2008. Visible symptoms caused by S. lautii and R. kalkhoffii on Colorado blue spruce took two growing seasons to develop, and the efficacy of the test products applied in 2008 could not be verified in the field until the fall of 2009. Field evaluations revealed there was no effect of the treatments on the 2009 foliage. However, five of the fungicide treatments significantly reduced levels of disease in the 2008 foliage.

Detection of Rhizosphaera infections in asymptomatic Colorado blue spruce needles was accomplished through polymerase chain reaction (PCR) analysis of DNA extracted from 2008 needles collected in the fall of 2008. Efforts to quantify the amount of pathogen present in the tissues in the laboratory, suggested that boscalid / pyraclostrobin provided the most control.

Overall, this screening study did not provide unequivocal results; what worked best at one site was not always mirrored at the other, and the laboratory evaluations did not support the field data. Although the results are not clear cut, they do suggest there would be value in conducting further work with propiconazole, pyraclostrobin and boscalid, and trifloxystrobin to find new solutions for control of needlecast diseases in conifers.

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