This review synthesizes the available knowledge on drought-disease interactions in forest trees with a focus on (1) evidence and patterns of drought-disease interactions, (2) current understanding of processes and mechanisms, and (3) three well documented cases studies. The first part is based on the analysis of a database of slightly more than one hundred studies, obtained by keyword searches combining drought, diseases or pathogens, and forest trees. A large majority of published studies referred to a positive association between drought and disease, i.e. disease favoured by drought or drought and disease acting synergistically on tree health status, with a predominance of canker/dieback diseases, caused by pathogens like Botryosphaeria, Sphaeropsis, Cytospora and Biscognauxia (Hypoxylon). The type of disease-related variables (incidence vs. severity) and the intensity and timing of water stress were shown to be significant factors affecting the drought- infection interaction. Interactions with other abiotic stresses and species-specific and genetic effects, related to host or pathogen, have also been reported. Direct effects of drought on pathogens are generally negative, although most fungal pathogens exhibit an important plasticity and can grow at water potentials well below the minimum for growth of their host plants. Studies on indirect effects of drought on pathogens through other community interactions are still relatively scarce. Positive drought-infection effects can mostly be explained by indirect effects of drought on host physiology. The predisposition and the multiple stress hypotheses are presented, as well as recent developments in the study of the molecular basis of abiotic and biotic stress, and their interactions. Sphaeropsis sapinea on pines, Biscognauxia mediterranea on oaks and root pathogens in declines associated with drought provide illustrative examples, treated as case studies, of pathogens of current significance associated with drought. The conclusion highlights some knowledge gaps, e.g. the role of latent parasites and the shift to a pathogenic stage, or the genetics of some fungal groups. The need for prevention of pathogen dispersal, especially crucial in the case of latent pathogens, is emphasized.

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Interactive effects of drought and pathogens in forest trees

Predisposition, Stress, and Plant Disease

Defence mechanisms against fungi in bark, xylem, roots, and foliage of woody angiosperms and gymnosperms are addressed in this newest addition to the "Springer Series in Wood Science". The topic is approached from an anatomical and physiological angle, with emphasis on the relationships between plant structure and function and their role in defensive reactions and wound repair. Contributions on the biochemistry of defensive reactions, in-depth treatises on specific diseases such as Dutch elm disease, and Fusiform rust of pines, as well as chapters on wound reactions in bark tissues, microenvironmental aspects of xylem decay, the role of mycorrhizal fungi in defence, physiological responses of trees to fungal symbionts of insects, and the application of tissue culture and immunocytochemistry to the study of defense mechanisms in trees are included.

Defense Mechanisms Of Woody Plants Against Fungi

The aggressiveness of Sphaeropsis sapinea isolates was compared on water-stressed and nonstressed 3-year-old red pines (Pinus resinosa) in greenhouse and growth chamber experiments. Water was withheld from stressed seedlings to achieve mean predawn needle water potentials (psi(PD)) above -1.9 MPa. The lowest mean psi(PD) of well-watered seedlings was maintained at or above -0.8 MPa. Young shoots were inoculated by placing colonized agar plugs on wounds made by removing a needle fascicle. Two isolates of each recognized morphotype (A and B) were used in the greenhouse experiment and two isolates of morphotype A were used in the growth chamber experiment. After 4 weeks, isolates of morphotype A caused more severe symptoms and could be recovered farther from the inoculation site on water-stressed than on nonstressed trees in both experiments. In the greenhouse experiment, isolates of mor-photype A also caused more severe symptoms and could be recovered farther from the inoculation site than isolates of morphotype B, regardless of watering regime. These results indicate that water stress at levels observed typically in the field can result in increased disease development by isolates of S. sapinea morphotype A on red pine. The reduction of water stress of red pines in the field may reduce losses due to Sphaeropsis shoot blight.

D.K. Bagga

Papers

The development of Hypoxylon canker of Populus tremuloides: role of interacting environmental factors