Janna L. Beckerman

Identification of resistance to multiple fungicides in field populations of Venturia inaequalis.

Venturia inaequalis, the causal agent of apple scab, is controlled primarily by fungicides. Long-term, extensive fungicide use has led to the development of resistance to multiple fungicides. To assess fungicide resistance, isolates of V. inaequalis were collected from Indiana and Michigan orchards. Single-spore derived isolates were evaluated by mycelium growth assays with previously determined discriminatory doses on media containing dodine, kresoxim-methyl, myclobutanil, or thiophanate-methyl. Of 195 isolates tested, 5.2, 0.7, 57.0, and 92.6% of isolates were found to be resistant to dodine, kresoxim-methyl, myclobutanil, and thiophanate-methyl, respectively. This is the first report of kresoxim-methyl field resistance in these states. Isolates resistant or shifted to a single fungicide were often found to have multiple fungicide resistance. Of all isolates tested, 38% were identified as resistant or shifted to two fungicides, and 12% were resistant or shifted to all four fungicides tested. No fitness penalty was found for isolates resistant to multiple fungicides based on a statistical analysis of mycelial growth and conidial production.

A Survey of Phytophthora spp. in Midwest Nurseries, Greenhouses, and Landscapes

A survey of nurseries, greenhouses, and landscapes was conducted from 2006 to 2008 in order to determine the prevalence and diversity of Phytophthora spp. From sites in Iowa, Michigan, Ohio, and, predominantly, Indiana, 121 Phytophthora isolates were obtained from 1,657 host samples spanning 32 host genera. Based on sequence of the internal transcribed spacer (ITS) region of the ribosomal DNA, 11 Phytophthora spp. and two hybrid species were identified. A majority of the isolates were P. citricola (35.9%) or P. citrophthora (27.4%). Six isolates were confirmed as hybrids (four of P. cactorum × hedraiandra and two of P. nicotianae × cactorum) by cloning and sequencing the ITS region. Three P. cactorum × hedraiandra isolates were obtained from the same site, from three Rhododendron spp., which are known hosts to the parental species. The fourth isolate, however, was recovered out of a different location in a Dicentra sp., which is not a known host to either parental species, suggesting an expansion of host range of the hybrid isolate as compared with either parental species.

A rapid method to quantify fungicide sensitivity in the brown rot pathogen Monilinia fructicola.

Management of brown rot of stone fruit relies upon the application of effective fungicides that may be compromised by the development of fungicide resistance. We evaluated fungicide resistance in the brown rot pathogen, Monilinia fructicola, using Alamar blue (AB) dye, or resazurin, a chromogenic substrate that can be used as an indicator of respiration, in a 96-well microtiter format. We compared the AB method to traditional mycelial growth assays for resistance screening using 10 isolates of M. fructicola that represented a range of sensitivities to fenbuconazole. Using traditional mycelial growth assays, isolate sensitivity ranged from 17.7 to 115.3% growth on medium amended with fenbuconazole at 0.03 μg/ml relative to that on nonamended medium. Concordant results between both assays were obtained (R2 = 0.9943, P < 0.0001), but the AB method provided results within 24 h, as opposed to the 3- to 5-day period required for mycelial growth assays. We found that sensitive isolates reduced AB less than resistant isolates in the presence of fungicide. Spore density influenced the reduction of AB by M. fructicola; spectrophotometric discrimination of fungicide sensitivity was best achieved at a density of 105 spores/ml.

In Situ Detection of Benzimidazole Resistance in Field Isolates of Venturia inaequalis in Indiana

Venturia inaequalis, the causal agent of apple scab, infects both commercial apples and ornamental crabapples. We found four classes of benzimidazole fungicide sensitivity in the Indiana population: sensitive (S) isolates unable to grow on 0.5 μg active ingredient (a.i.)/ml; low resistant (LR) isolates that grew at 0.5 μg a.i./ml, but not at 5 μg a.i./ml; moderately resistant (MR) isolates that grew at 5 μg a.i./ml, but not at 50 μg a.i./ml; and very highly resistant (VHR) isolates that grew rapidly at 50 μg a.i./ml. Polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) of the β-tubulin gene with two restriction enzymes, BstUI and Cac8I, enabled us to rapidly identify benzimidazole resistance among all tested isolates. Sixty-nine percent of the resistant isolates tested possessed the BstUI RFLP at codon 198 that corresponds to VHR, and the remaining LR and MR isolates possessed the Cac8I RFLP corresponding to a newly identified resistance allele at codon L240F. Combined, PCR-RFLP correctly identified the resistance status of all isolates tested to date. The preponderance of benzimidazole-resistant isolates from commercial apple orchards and their absence in the landscape on ornamental crabapple suggests that two distinct populations of V. inaequalis coexist in Indiana.

Incorporating adjuvants with captan to manage common apple diseases.

Captan has become an increasingly important fungicide in the management of common apple diseases such as apple scab (Venturia inaequalis) and bitter rot (Colletotrichum spp.) due to the low risk of fungicide resistance evolving in either pathogen population to this product. Restrictions on the amount of captan that can be applied per season limits the amount and the number of applications a grower may use, resulting in control failures during high disease pressure years. This 3-year field study evaluated how adjuvants combined with captan affected the incidence and severity of apple scab and bitter rot on two different apple cultivars. Results showed that Li700 plus captan and Bond Max plus captan reduced disease incidence of apple scab and bitter rot in years with moderate to high disease pressure by increasing the coverage and retention of captan. The addition of these adjuvants also resulted in possible yield losses due to russetting caused by phytotoxicity. The overall benefits of incorporating adjuvants with captan based on this study could reduce disease incidence while potentially saving a grower up to

Towards a Sustainable, Integrated Management of Apple Diseases

5,329 ha-1 due to fungicide reduction.

Occurrence of QoI Resistance and Detection of the G143A Mutation in Michigan Populations of Venturia inaequalis

Current understandings and guidelines for sustainable, integrated management of apple diseases are reviewed, and currently effective and sustainable tactics are discussed. Disease management in apples faces several critical problems not seen in other agronomic systems. As long-lived, clonal crops, fungicides and disease resistance play crucial roles in the management of apple diseases. Unfortunately, the pressure placed on these two strategies results in the development of fungicide resistance, and breakdown of host resistance. This review discusses the major diseases of apples, their management strategies, and the problems that have developed to impact sustainable apple production. Symptoms, causal pathogens, disease cycles and management practices are reviewed for primary diseases affecting apples in spring like apple scab, powdery mildew, fire blight and rust diseases. Problems due to fungicide resistance and availability of root stocks and cultivars for exploitation of plant resistance are discussed. Applications of cultural and chemical management with predictive models are also shown. Symptoms, disease cycles and management are also reviewed for summer diseases of apple, like bitter rot, flyspeck and sooty blotch