Threats to global food security from emerging fungal and oomycete crop pathogens.

Emerging fungal and oomycete pathogens infect staple calorie crops and economically important commodity crops, thereby posing a significant risk to global food security. Our current agricultural systems - with emphasis on intensive monoculture practices - and globalized markets drive the emergence and spread of new pathogens and problematic traits, such as fungicide resistance. Climate change further promotes the emergence of pathogens on new crops and in new places. Here we review the factors affecting the introduction and spread of pathogens and current disease control strategies, illustrating these with the historic example of the Irish potato famine and contemporary examples of soybean rust, wheat blast and blotch, banana wilt and cassava root rot. Our Review looks to the future, summarizing what we see as the main challenges and knowledge gaps, and highlighting the direction that research must take to face the challenge of emerging crop pathogens.

Rapid loss of virulence during submergence of Z. tritici asexual spores.

Zymoseptoria tritici, the causal agent of Septoria tritici blotch, is a notable pathogen of temperate-grown wheat. To better understand the mechanisms underpinning pathogenicity, leaf infection assays are commonly used to compare either the virulence of Z. tritici wildtype or mutant strains, or the susceptibility of wheat cultivars. These assays, which control for many biotic, abiotic and experimental variables, involve the application of known spore numbers to leaves. To achieve this, spore numbers are quantified during a period of aqueous suspension. Published methods rarely state the period in which spores are held in suspension, suggesting that this variable may be uncontrolled. Using simple, agar-based plating experiments, this work firstly demonstrates that blastospore culturability (the ability to form a colony when plated on appropriate agar) decreases rapidly over time during maintenance in aqueous suspension. It is subsequently shown that this reduction in culturability correlates to a reduction in the virulence of the blastospore population. This is shown in three wild type Z. tritici strains. From this, it is concluded that suspension time is a variable of major importance in experimental design and one which, if not controlled, may lead to erroneous conclusions from inter-strain comparisons. The conidia of the unrelated fungus Magnaporthe oryzae also rapidly lose culturability when stored in aqueous suspension, whereas the microspores of Fusarium oxysporum f. sp. cubense do not, suggesting that this phenomenon occurs in some but not all other fungi. Finally, a droplet method of inoculations is proposed to decrease the variability in the numbers of spores applied, within and between experiments.