Phytotoxins Produced by Diplodia and Other Fungal Species Involved in Oak (Quercus) Diseases

Forests are an indispensable resource for human existence in the different regions of the world. Forests are often concentrated in a few parts and vary in size. The most important and extensive green reserve on Earth is the Amazon rainforest. Unfortunately, the forests are continually suffering massive losses due to abiotic stresses and deforestation to obtain other arable lands and microbial diseases, particularly those caused by pathogenic fungi. Pathogenic fungi play an important role in inducing oak decline, which is one of the most relevant forest diseases globally. The pathogens also affect disease development and its global distribution. In this review, the phytotoxins produced by fungal pathogens of different oak species in various world regions are described in depth. In particular, the isolation, the chemistry and biology of phytotoxins synthesized by oak pathogenic fungi are reported, and their role in pathogenesis is also discussed. For some phytotoxins, the enantioselective synthesis, the structure-biological activity relationships, and their utilization in agro-industry and medical applications are also discussed. In addition, for the Diplodia species, the comparison between their secondary metabolite profile and taxonomy with those of fungal pathogens of other close forest plants is discussed. Thus, fungal diseases of numerous oak trees and the serious damages they cause to the forest heritage are described, as well as the chemical and biological properties and results of some of the structure-activity relationship (SAR) studies of the phytotoxins involved. In addition, for the Diplodia species both phylogenetically closely related and unrelated, comparison between their secondary metabolite profile with those of pathogens of other close forest plants, in combination with traditional taxonomic characteristics, has helped to better understand the pathogenic process relationships within the same fungal f the pathogenic process within the same fungal family.