Screening of fungicides for the control of leaf spot diseases in lowbush blueberry

Project Code SCR08-020

Project Lead

Paul D. Hildebrand - Agriculture and Agri-FoodCanada

Objective

The objective of this study was to assess the efficacy of a number of reduced risk products for their potential use in the control of Septoria leaf spot, rust and Valdensinia leaf spot in lowbush blueberry

Summary of Results

Background

Lowbush blueberry is affected by three major leaf spot diseases that can cause yield loss. These diseases are Septoria leaf spot (Septoria sp.), rust (Naohidemyces vaccinia also known as Thekopsora minima, or Pucciniastrum vaccinii) and Valdensinia leaf spot (Valdensinia heterodoxa). Foliar infections of these fungi on sprouts result in premature leaf drop in the sprout year leading to reduced fruit bud development. Infections in the fruiting year can lead to reduced fruit size. Rust appears to be less severe in the fruiting year because the disease does not usually reach high levels until after harvest. Valdensinia leaf spot develops at any time of the season and is highly dependent on weather conditions. Valdensinia infections on the leaves of sprout stems cause premature defoliation of lowbush blueberry.

Chlorothalonil is the only product registered for the control of Septoria, rust and valdensinia. While this product is useful in sprout fields, it has a 54 day pre-harvest interval which may restrict its use in fruiting fields. The goal of this project was to assess the efficacy of a number of reduced risk products for their potential use in the control of leaf spot and rust diseases in lowbush blueberry.

Approaches

Screening trials were initiated in 2009 in Nova Scotia. One biological fungicide and several chemical fungicides were evaluated for the control of Septoria leaf spot and leaf rust in lowbush blueberry. Plots were arranged in a randomized complete block design with 5 or 6 replicates in fields either in the fruiting or sprouting phase of production. Two untreated controls were also included in each replicate. Fungicides were applied in 600 litres of water per hectare with a hand held pressurized carbon dioxide sprayer using a Cone Jet TXVS-10 hollow cone nozzle operating at 276 kilopascals. Disease severity, incidence of defoliation and yields were measured. Signs of phytotoxicity were also assessed after fungicide application.

Results

Results from this screening trial showed that the biopesticide did not provide control of Septoria leaf spot and leaf rust in lowbush blueberry.

Of the chemical fungicides included in the screening trial, Proline (prothioconazole), Pristine (pyraclostrobin and boscalid), Absolute (tebuconazole and trifloxystrobin) and Quilt (azoxystrobin and propiconazole) provided good disease control, reduced defoliation and increased yield and were identified as the strongest candidates for control of Septoria leaf spot. The fungicides Proline (prothioconazole), Eminent (tetraconzole) and Quilt (azoxystrobin and propiconazole) were identified as the strongest candidates for control of leaf rust. Bravo (chlorothalonil), which is already registered for use on blueberry in Canada, was also included in the trials and effectively controlled both Septoria leaf spot and leaf rust. In trials to evaluate potential candidates for management of Valdensinia leaf spot, several fungicides including Pristine, Absolute and Lance, were identified as candidates for further evaluation.

 

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