Phoma stem canker (blackleg), caused by Leptosphaeria maculans, is an important disease on oilseed rape (canola, rapeseed, Brassica napus, Brassica juncea, Brassica rapa) causing seedling death, lodging or early senescence in Australia, Canada and Europe, but not in China. The two forms of L. maculans (A group and B group) that occur on oilseed rape are now considered to be separate species. The epidemiology and severity of phoma stem canker differs between continents due to differences in the pathogen population structure, oilseed rape species and cultivars grown, climate and agricultural practices. Epidemics are most severe in Australia, where only the A group occurs, and can be damaging in Canada and western Europe, where both A and B groups occur, although their proportions vary within regions and throughout the year. Epidemics are slight in China, where the A group has not been found. Dry climates (Australia, western Canada) lengthen the persistence of infected debris and may synchronize the release of airborne ascospores (after rain) with seedling emergence. L. maculans spreads from cotyledon and leaf infections down petioles to reach the stem, with infections on cotyledons and leaves early in the season producing the most damaging stem cankers at the stem base (crown). Development of both crown cankers and phoma stem lesions higher up stems is most rapid in regions with high temperatures from flowering to harvest, such as Australia and Canada. Breeding for resistance (genetic, disease escape or tolerance), stubble management, crop rotation and fungicide seed treatments are important strategies for control of phoma stem canker in all areas. Fungicide spray treatments are justified only in regions such as western Europe where high yields are obtained, and accurate forecasts of epidemic severity are needed to optimize their use.
Strobilurin fungicides or quinone outside inhibitors (QoIs) have been used successfully to control Septoria leaf blotch in the United Kingdom since 1997. However, QoI-resistant isolates of Mycosphaerella graminicola were reported for the first time at Rothamsted during the summer of 2002. Sequence analysis of the cytochrome b gene revealed that all resistant isolates carried a mutation resulting in the replacement of glycine by alanine at codon 143 (G143A). Extensive monitoring using real-time polymerase chain reaction (PCR) testing revealed that fungicide treatments based on QoIs rapidly selected for isolates carrying resistant A143 (R) alleles within field populations. This selection is driven mainly by polycyclic dispersal of abundantly produced asexual conidia over short distances. In order to investigate the role of sexually produced airborne ascospores in the further spread of R alleles, a method integrating spore trapping with real-time PCR assays was developed. This method enabled us to both quantify the number of M. graminicola ascospores in air samples as well as estimate the frequency of R alleles in ascospore populations. As expected, most ascospores were produced at the end of the growing season during senescence of the wheat crop. However, a rapid increase in R-allele frequency, from 35 to 80%, was measured immediately in airborne ascospore populations sampled in a wheat plot after the first QoI application at growth stage 32. After the second QoI application, most R-allele frequencies measured for M. graminicola populations present in leaves and aerosols sampled from the treated plot exceeded 90%. Spatial sampling and testing of M. graminicola flag leaf populations derived from ascospores in the surrounding crop showed that ascospores carrying R alleles can spread readily within the crop at distances of up to 85 m. After harvest, fewer ascospores were detected in air samples and the R-allele frequencies measured were influenced by ascospores originating from nearby wheat fields.
This review considers factors affecting the coexistence of closely related pathogen species on arable crops, with particular reference to data available at Rothamsted for Septoria tritici/Stagonospora nodorum (Mycosphaerella graminicola/Phaeosphaeria nodorum) (septoria leaf blotch diseases on winter wheat), Oculimacula yallundae/O. acuformis (eyespot disease of winter cereals), and Leptosphaeria maculans/L. biglobosa (phoma stem canker on winter oilseed rape). Factors affecting the short-term, medium-term, and long-term coexistence of such related pathogen species are reviewed, and their evolution from common ancestors considered. Small niche differences between the related pathogen species enable them to coexist on the same host. The niche differences result from small differences in their biology/epidemiology, leading to separation in space, time, or resource use. Changes in both natural (e.g., fluctuating temperature) and man-made (e.g., agronomic practices, pollution) factors influence the coexistence. Such factors may result in coexistence between the related species in some parts of the world, whereas in other parts only one species occurs. These principles illustrated with pathogens of arable crops are generic to other host-pathogen systems.
The colonization of winter oilseed rape plants and epidemiology of phoma stem canker differed between A/Tox + and B / Tox 0 Leptosphaeria maculans . In France and England, where plant colonization was investigated during two and three growing seasons, respectively, there was a difference in timing of leaf infection; A / Tox + L. maculans was predominant on leaves in the autumn (October / November) but there was an increase in the incidence of B / Tox 0 in the winter ( January/ February). In May, June and July both species could be isolated from all external parts of the plant (root to the upper stem) and all crown (stem base) tissues, although they differed in their distribution. At the root and crown, A / Tox + L.
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