Effects of cherry leaf spot on photosynthesis in tart cherry 'Montmorency' foliage.

Results described here span a total of three field seasons and quantitatively depict the effects of an economically important fungal pathogen (Blumeriella jaapii) on tart cherry (Prunus cerasus 'Montmorency') leaf physiology. For the first time, leaf photosynthesis, stomatal conductance (g(s)), maximum ribulose-1,5-bisphosphate carboxylation rate (V(cmax)), and maximum electron transport (J(max)) were measured as functions of visible cherry leaf spot disease (CLS) severity. Defined as the proportion of chlorotic and necrotic tissue per leaf, CLS severity was estimated from leaves of mature 'Montmorency' trees in 2007, 2008, and 2009. Briefly, as visible disease severity increased, all of the leaf-level physiological parameters decreased significantly (P < 0.01) and disproportionately. Thus, the effects of visible symptoms on leaf photosynthetic metabolic function encroached upon asymptomatic tissue as well. Impairment of photosynthetic metabolism in 'Montmorency' tart cherry leaves due to CLS appears to be mediated through disproportionately large perturbations in g(s), V(cmax), and J(max). These findings offer a new perspective on the amount of damage that this serious disease can inflict.

Effects of Copper-Based Fungicides on Foliar Gas Exchange in Tart Cherry.

In the Great Lakes region of the United States, cherry growers are poised to re-adopt copper-based fungicides to manage cherry leaf spot disease (CLS), caused by Blumeriella jaapii. However, application of copper is often associated with leaf bronzing. In growth chamber experiments, bronzing was observed on foliage of tart cherry (Prunus cerasus 'Montmorency') seedlings 1 week following application of a copper-based fungicide, only when leaves were also exposed to nightly dew. In potted, 1-year-old trees outdoors, light-saturated rates of net CO2 assimilation (A) and stomatal conductance (gs) were not affected by treatment with copper sulfate, chlorothalonil, tebuconazole, or trifloxystrobin compared to a nonsprayed control. In 2005 and 2006, A and gs were measured during late summer on leaves of mature trees in an orchard subjected to the following fungicide programs: synthetic fungicides only; synthetic fungicides integrated with copper-based fungicides; or not sprayed. Bronzing symptoms were observed on trees sprayed with copper. Regression analysis revealed that neither A nor gs decreased as leaf surface area affected by bronzing increased (R2 = 0.004, P = 0.80 and R2 = 0.006, P = 0.74, respectively). Leaf bronzing associated with application of copper-based fungicides may therefore be inconsequential to foliar gas exchange in tart cherry during late summer.