Hemibiotrophic pathogens cause significant losses within agriculture, threatening the sustainability of food systems globally. These microbes colonise plant tissues in three phases: a biotrophic phase followed by a biotrophic-to-necrotrophic switch phase and ending with necrotrophy. Each of these phases is characterized by both common and discrete host transcriptional responses. Plant hormones play an important role in these phases, with foliar models showing that salicylic acid accumulates during the biotrophic phase and jasmonic acid/ethylene responses occur during the necrotrophic phase. The appropriateness of this model to plant roots has been challenged in recent years. The need to understand root responses to hemibiotrophic pathogens of agronomic importance necessitates further research. In this study, using the root hemibiotroph Phytophthora medicaginis, we define the duration of each phase of pathogenesis in Cicer arietinum (chickpea) roots. Using transcriptional profiling, we demonstrate that susceptible chickpea roots display some similarities in response to disease progression as previously documented in leaf plant–pathogen hemibiotrophic interactions. However, our transcriptomic results also show that chickpea roots do not conform to the phytohormone responses typically found in leaf colonisation by hemibiotrophs. We found that quantified levels of salicylic acid concentrations in root tissues decreased significantly during biotrophy while jasmonic acid concentrations were significantly induced. This study demonstrated that a wider spectrum of plant species should be investigated in the future to understand the physiological changes in plants during colonisation by soil-borne hemibiotrophic pathogens before we can better manage these economically important microbes.

SUMMARY: The extent and severity of wheat take-all (caused byGaeumannomyces graminisvar.tritici(Ggt)) can vary considerably between growing seasons. The current study aimed to identify climatic factors associated with differing concentrations ofGgtDNA in soil and take-all disease at different stages of a sequence of wheat crops. Pre-sowing soilGgtDNA concentrations and subsequent take-all disease in consecutive wheat crop sequences were compared across six seasons in 90 commercial cropping fields in Canterbury and Southland, New Zealand, between 2003 and 2009. Disease progress was assessed in additional fields in 2004/05 and 2005/06. While a general pattern in inoculum and disease fluctuations was evident, there were exceptions among wheat crop sequences that commenced in different years, especially for first wheat crops. In three consecutive growing seasons, there was very low inoculum increase in the first wheat crop, while increases in first wheat crops during the following three seasons was much greater. Low spring–summer rainfall was associated with low build-up of inoculum in first wheat crops. The inoculum derived from the first wheat then determined the amount of primary inoculum for the subsequent second wheat, thereby influencing the severity of take-all in that crop. Differing combinations of weather conditions during one wheat crop in a sequence and the conditions experienced by the next crop provided explanations of the severity of take-all at grain fill and the resulting post-harvest soilGgtDNA concentrations in second wheat crops. Examples of contrasting combinations were: (a) a moderate take-all epidemic and high post-harvest inoculum that followed high rainfall during grain fill, despite low pre-sowing soilGgtDNA concentrations; (b) severe take-all and moderate to high inoculum build-up following high pre-sowing soilGgtDNA concentrations and non-limiting rainfall; and (c) low spring and early summer rainfall slowing epidemic development in second wheat crops, even where there were high pre-sowing soilGgtDNA concentrations. The importance of the environmental conditions experienced during a particular growing season was also illustrated by differences between growing seasons in take-all progress in fields in the same take-all risk categories based on pre-sowing soilGgtDNA concentrations.

Phytophthora medicaginis causing Phytophthora root rot of chickpea ( Cicer arietinum ) is an important disease, with genetic resistance using C . arietinum × Cicer echinospermum crosses as the main disease management strategy. We evaluated pathogenic variation in P . medicaginis populations with the aim of improving phenotyping methods for disease resistance. We addressed the question of individual isolate aggressiveness across four different seedling‐based phenotyping methods conducted in glasshouses and one field‐based phenotyping method. Our results revealed that a seedling media surface inoculation method used on a susceptible C . arietinum variety and a moderately resistant C . arietinum × C . echinospermum backcross detected the greatest variability in aggressiveness among 37 P . medicaginis isolates. Evaluations of different components of resistance, using our different phenotyping methods, revealed that differential pathogen–isolate reactions occur with some phenotyping methods. We found support for our hypotheses that the level of aggressiveness of P . medicaginis isolates depends on the phenotyping method, and that phenotyping methods interact with both isolate and host genotype reactions. Our cup‐based root inoculation method showed promise as a non‐field‐based phenotyping method, as it provided significant correlations with genotype–isolate rankings in the field experiment for a number of disease parameters.