Optimizing the Timing of the First Fungicide Application to Manage Cranberry Fruit Rot

Cranberry fruit rot is a major constraint to cranberry production and is managed using season-long fungicide programs. A series of replicated field experiments conducted over two study periods (1996–2000 and 2002–2007), together with supplemental commercial validation trials, examined how the timing of the first fungicide application relative to host phenology influences disease control. Each year, progressively delayed applications of chlorothalonil were initiated during bloom. Application timing was normalized across years using a phenological reference point corresponding to 60% out-of-bloom. Across both datasets, loss of disease control followed a significant nonlinear relationship with application timing. Disease control declined gradually when the first application was initiated prior to late bloom but decreased sharply when initiation was delayed into late bloom and early fruit set. In the more densely sampled 2002–2007 dataset, a second-order polynomial model explained 42% of the variation in disease control (R² = 0.42; P < 0.001; n = 55) and resolved a rapid collapse in efficacy associated with delayed application timing. Analysis of the 1996–2000 dataset independently confirmed the similar phenological response pattern (R² = 0.61; P = 0.0001; n = 24), despite coarser temporal resolution. Together, these results identify a reproducible phenological window during which infection of cranberry fruit greatly increases the probability of rot development. Collectively, the findings demonstrate that CFR expression is governed primarily by host developmental biology rather than by prolonged seasonal pathogen pressure or pathogen species composition, providing a biological foundation for reduced-input fungicide programs focused on precisely timing the first application.