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Southern California experienced unprecedented megadrought between 2012 and 2018. During this time, Malosma laurina, a chaparral species normally resilient to single‐year intense drought, developed extensive mortality exceeding 60% throughout low‐elevation coastal populations of the Santa Monica Mountains. We assessed the physiological mechanisms by which the advent of megadrought predisposed M. laurina to extensive shoot dieback and whole‐plant death. We found that hydraulic conductance of stem xylem (Ks, native) was reduced seven to 11‐fold in dieback adult and resprout branches, respectively. Staining of stem xylem vessels revealed that dieback plants experienced 68% solid‐blockage, explaining the reduction in water transport. Following Koch's postulates, persistent isolation of a microorganism in stem xylem of dieback plants but not healthy controls indicated that the causative agent of xylem blockage was an opportunistic endophytic fungus, Botryosphaeria dothidea. We inoculated healthy M. laurina saplings with fungal isolates and compared hyphal elongation rates under well‐watered, water‐deficit, and carbon‐deficit treatments. Relative to controls, we found that both water deficit and carbon‐deficit increased hyphal extension rates and the incidence of shoot dieback. Summary statement: We conclude that the ultimate cause of mortality of Malosma laurina in field populations was megadrought and that the proximal cause of mortality was the transition of endemic Botryosphaeria dothidea to an opportunistic pathogen that amplified protracted water stress through physical blockage of xylem water transport. [ABSTRACT FROM AUTHOR]

Neonicotinoid seed treatment (NST) is a routine practice used worldwide to control insect pests in a variety of crops, including maize (Zea mays mays L.). However, previous work indicates that systemic insecticides can compromise plant defenses, counteracting efforts to control insect pests. The goal of this study was to evaluate the effect of thiamethoxam-neonicotinoid seed treatment on the resistance of two maize genotypes (B73 and MC 4050) against the major non-target pest, fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae). In preference and performance assays, we evaluated the effect of NST on fall armyworm behavior and biology. We also determined the influence of NST on induced plant defenses, quantifying phytohormone levels and plant volatile emissions, in treatments with and without fall armyworm herbivory. NST did not affect caterpillar host preference, however it reduced caterpillar performance on the genotype B73 across both maize growth stages (V4 and V6). NST-treated B73 plants also had lower induced volatile production (V4 stage) compared to untreated herbivore-damage plants and lower constitutive salicylic acid (V6 stage). In contrast, MC 4050 was not affected by the insecticide, regardless of growth stage. In conclusion, we found that the effects of NST on maize defenses vary by plant genotype and growth stage, suggesting growers may need to tailor their selection of plant genotypes to avoid negative impacts of NST on plant resistance and ultimately pest control. [ABSTRACT FROM AUTHOR]