Sensitivity of Phacidiopycnis spp. Isolates from Pome Fruit to Six Pre- and Postharvest Fungicides.

Phacidiopycnis washingtonensis and P. pyri cause speck rot and Phacidiopycnis rot on apple and pear, respectively. Infection occurs in the orchard and remains latent, and symptoms appear after months of storage. Decay management relies on orchard sanitation and pre- and postharvest fungicides. In a 2017 survey, speck rot accounted for 6.4% of apple decay in central Washington, whereas Phacidiopycnis rot accounted for 3.9 and 6.7% of total pear decay in Washington and Oregon, respectively. Sensitivities of baseline populations of 110 P. washingtonensis and 76 P. pyri isolates collected between 2003 and 2005 to preharvest fungicides pyraclostrobin (PYRA) and boscalid (BOSC) and to postharvest fungicides thiabendazole (TBZ), fludioxonil (FDL), pyrimethanil (PYRI), and difenoconazole (DFC) were evaluated using a mycelial growth inhibition assay. Mean effective concentrations necessary to inhibit 50% growth (EC50) of P. washingtonensis were 0.1, 0.3, 0.8, 1.8, 2.1, and 4.8 µg/ml for FDL, PYRI, TBZ, DFC, PYRA, and BOSC, respectively. Respective mean EC50 values for P. pyri were 0.2, 0.6, 1.6, 1.1, 0.4, and 1.8 µg/ml. The sensitivity of exposed P. washingtonensis and P. pyri populations collected in 2017 revealed potential shifts toward BOSC and PYRA resistance. The efficacy of the six fungicides to control isolates of each pathogen with different in vitro sensitivity levels was evaluated on apple and pear fruit. FDL, DFC, and PYRI controlled both Phacidiopycnis spp. regardless of their EC50 values after 5 months of storage at 0°C in a regular atmosphere. The consistent occurrence of Phacidiopycnis spp. will require continuous monitoring and development of disease management strategies based on fungicide phenotypes and efficacy of existing fungicides assessed herein.

Study of Fitness, Virulence, and Fungicide Sensitivity of Lambertella corni-maris Causing Yellow Rot on Apple.

Very little is known about the biology, epidemiology, and best practices to manage Lambertella rot, a newly identified postharvest disease caused by Lambertella corni-maris on apples in the United States. In this study, we investigated the prevalence of L. corni-maris in 92 grower lots throughout Washington State in 2016, evaluated the effect of nutrient availability on L. corni-maris growth, and L. corni-maris sensitivity to pH, osmotic stress, and fungicides in vitro. We assessed pathogen aggressiveness on major apple cultivars and the efficacy of pre- and postharvest fungicides to control L. corni-maris on detached fruit. L. corni-maris was widespread and was found in 40% of the growers lots surveyed at frequencies ranging from 2 to 40% of the total decay. The fungus grew faster on acidic media such as apple juice agar and V8 agar media and was able to grow equally at pH values ranging from 3 to 7. L. corni-maris isolates showed relatively low sensitivity to osmotic stress and grew evenly at 1M KCl. All nine apple cultivars tested were susceptible to L. corni-maris, but the disease severity was significantly higher on some cultivars such as Honeycrisp, Piñata, and Gala. The baseline sensitivity of 100 L. corni-maris isolates was determined based on the effective concentration necessary to inhibit 50% mycelial growth (EC50) for fludioxonil, boscalid, pyraclostrobin + SHAM, pyrimethanil, and thiabendazole. The respective mean EC50 values were 0.07, 0.84, 1.32, 2.45, and 3.68 µg/ml. Fludioxonil and pyrimethanil applied at label rates were the only fungicides able to control L. corni-maris effectively on detached apple fruit. This study is a first step toward understanding the eco-physiological requirements for L. corni-maris to survive and cause decay on apple, assessing potential shifts in fungicide sensitivity, and enhancing Lambertella rot management. We suggest "yellow rot" as the new common name for Lambertella rot.