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Extreme droughts are increasing in frequency, severity, and duration in arid and semi-arid regions around the world due to climate change. As a result, plant species that are typically capable of withstanding regular drought stress are exposed to conditions outside of their normal range, rendering them susceptible to opportunistic disease-causing agents. Theoretical frameworks describing the roles of environmental and biotic stressors in driving plant mortality are well established. However, there is a lack of empirical data with which to resolve how these factors interact in vivo. Furthermore, studies that document progression of stress and dieback throughout the course of a multi-year drought event in situ are rare. In this dissertation, I detail a series of studies aimed at understanding mechanisms of dieback and mortality by focusing on a severe canopy dieback event in a classically drought tolerant chaparral shrub, big berry manzanita (Arctostaphylos glauca) in Santa Barbara, California, during an historic California drought. I provide strong evidence that dieback is caused by members of the fungal Botryosphaeriaceae (Bot.) family in conjunction with extreme drought, and that dieback is also related to increased drought stress along an elevational gradient. By conducting a field survey, I identify Neofusiccocum australe as the most prevalent and widely distributed fungal pathogen in A. glauca., and that dieback is strongly correlated with Bot. infection. Using a full-factorial design in a greenhouse experiment, I provide evidence that extreme drought and infection by N. australe can indeed act synergistically, together driving faster and greater mortality in young (approx. one-year-old) A. glauca than either factor alone. Lastly, by taking measurements on water availability, dark-adapted leaf fluorescence, and photosynthesis in A. glauca shrubs across an elevational gradient, I provide evidence that landscape-level factors can contribute to localized variability in water stress and canopy dieback severity in A. glauca, and may be useful in predicting vulnerabilities during future drought. Remarkably, no new mortality was observed throughout the study, suggesting extreme resiliency in adult shrubs. However, canopy dieback alone can impact wildlife and fuel loads, even when not associated with mortality. Together, these results provide strong evidence that A. glauca dieback was caused synergistic effects between extreme drought and infection by N. australe, and that lower elevations and exposed slopes may be at greatest risk for future events.

Botryosphaeria dieback is a major trunk disease of grapevines worldwide that has been recognised for its importance during the last 15-20 years. This disease causes direct losses by killing canes, arms and vines, which ultimately reduce yields in the vineyards. There are five Botryosphaeriaceae species that commonly infect grapevines in New Zealand vineyards, although many more species have been reported worldwide. Reports on pathogenicity and epidemiology overseas have shown large variation between the predominant species in different countries and so this research aimed to improve understanding of spore production, dispersal and infection dynamics for the most common species within a Marlborough vineyard setting. Pathogenicity testing with multiple isolates of Neofusicoccum luteum and N. parvum showed that all isolates caused disease of different grapevine stem tissues, with great differences in symptoms and conidial release between isolates of each species. Overall trends showed that N. luteum caused longer lesions and produced more conidia than N. parvum and isolates of both species released conidia at all temperatures (10-25ºC) and relative humidities (80-100%), although maximum conidial release occurred at 25ºC and 100% RH. Cirrhi were also observed in vineyards in moist conditions. Rainwater run-off and Burkard spore traps collected Botryosphaeriaceae conidia in three Marlborough vineyards in 2013 and 2015. The conidia were found only during rainfall and up to 2 h after rainfall ceased, release occurring after as little as 0.2 mm rain and conidial numbers were generally greater for greater rainfall. Identification of species trapped by single stranded conformational polymorphism (SSCP) indicated presence of N. luteum, N. parvum/ribis, N. australe, D. mutila and D. seriata. Within vineyards, conidia of a marker isolate of N. parvum were shown to disperse during 2 days rain, up to 10 m in the wind direction and up to 5 m in other directions. Further, these conidia caused many infections of newly trimmed shoots up to 2 cm from the conidial source. Conidia of N. luteum and N. parvum were able to infect 100% of wounds on green shoots and trunks of potted vines for 7 days after wounding. Incidence decreased with increasing age of wounds, with hard shoots being susceptible until 56 days old and trunk wood until 28 days old. Incidences and colonisation distances were also affected by the seasons, being highest in autumn and spring, and lowest in winter and summer. Also different conidial numbers (2-100) affected incidences and colonisation distances on wounded stems of vines with as little as 2 conidia causing 100% infection incidence of wounded green shoots and 67% incidence of trunks. The pathogen infected the underlying tissues after spray application of conidia onto non-wounded cane and trunk bark, and progressed rapidly towards nearby wounds made later. Also infection through lenticels was observed by fluorescence microscopy. Wounded leaf buds, and berries on potted vines and in the vineyards were able to be infected at all stages of growth tested, but non-wounded buds and berries also become infected at late stages of development. Infection progressed from the buds into the developing green shoots and supporting canes, and from berries into the bunch stems and supporting canes. Experiments using a range of treatments to prevent infection of wounds in stem and trunk tissues of vines grown in pots and a in a vineyard showed that Cheif®, Megastar® and Folicur® were most effective at reducing infection by N. luteum and N. parvum, when inoculation was carried out 2 h, 7 and 14 days after treatment and with 2-600 conidia. The biological control product, TRI D25, was as effective as Megastar® when inoculation occurred 7-14 days after treatment, but only on potted vines. These treatments also reduced colonisation distances greatly within inoculated tissues which indicated potential for control by pruning. This research has provided new information on the disease cycle of Botryosphaeriaceae diseases in vineyards. It has elucidated effects of environmental conditions on release and dispersal of conidia. These studies also demonstrated the effects of some host, environmental and pathogen factors on infection of pruning and trimming wounds and disease development in the various types of stem tissues, and that infection may take place in non-wounded shoots of living vines. The potential infection of leaf buds and berries and sporulation from berries were also demonstrated. Some control products were shown to be effective in preventing infection but limitations on their use indicates the need for integrated control methods which also aim to reduce inoculum sources in vineyards.