A Single, Plastic Population of Mycosphaerella pinodes Causes Ascochyta Blight on Winter and Spring Peas (Pisum sativum) in France

Abstract

c Plant diseases are caused by pathogen populations continuously subjected to evolutionary forces (genetic flow, selection, and recombination). Ascochyta blight, caused by Mycosphaerella pinodes, is one of the most damaging necrotrophic pathogens of field peas worldwide. In France, both winter and spring peas are cultivated. Although these crops overlap by about 4 months (March to June), primary Ascochyta blight infections are not synchronous on the two crops. This suggests that the disease could be due to two different M. pinodes populations, specialized on either winter or spring pea. To test this hypothesis, 144 pathogen isolates were collected in the field during the winter and spring growing seasons in Rennes (western France), and all the isolates were genotyped using amplified fragment length polymorphism (AFLP) markers. Furthermore, the pathogenicities of 33 isolates randomly chosen within the collection were tested on four pea genotypes (2 winter and 2 spring types) grown under three climatic regimes, simulating winter, late winter, and spring conditions. M. pinodes isolates from winter and spring peas were genetically polymorphic but not differentiated according to the type of cultivars. Isolates from winter pea were more pathogenic than isolates from spring pea on hosts raised under winter conditions, while isolates from spring pea were more pathogenic than those from winter pea on plants raised under spring conditions. These results show that disease developed on winter and spring peas was initiated by a single population of M. pinodes whose pathogenicity is a plastic trait modulated by the physiological status of the host plant. P lant parasites can quickly adapt to their hosts and overcome resistance genes used in crop protection. This process occurs both in agroecosystems, where hosts (cultivars) are typically quite uniform genetically, and in wild pathosystems, where hosts and parasites show high levels of diversity and are structured as metapopulations (12). However, a number of crop plants are grown in complex ecosystems rather than as the monocultures which characterize modern agriculture; this is particularly the case for species grown as both winter and spring cultivars and for which wild relatives occur as weeds or in field margins.From a plant pathology point of view, such complex ecosystems are usually characterized by one or more of the following parameters: (i) spatial and temporal heterogeneity of hosts, which creates patchiness and an often complex age structure, (ii) intraand interspecific diversity, and (iii) multiple host-pathogen interactions. These features favor niche partitioning, as they allow the coexistence on the same host species of different pathogen populations separated in space and/or time. Indeed, ecological differences that lead to niche partitioning can occur in three basic ways: resource partitioning, temporal niche partitioning, and spatial niche partitioning (4, 14, 36). For plant-pathogenic species able to exploit the same host, separation in space and/or time mi...

Seasonal Changes Drive Short-Term Selection for Fitness Traits in the Wheat Pathogen Zymoseptoria tritici

“…This was consistent with the increase in disease intensity recorded in different fungal pathogens after only a few asexual cycles (29)(30)(31)(32). Isolates of the final population (Pf) showed greater sporulation intensity under winter conditions, and a shorter latency period under spring conditions, than isolates of the initial population (Pi).…”

Aggressiveness Changes in Populations of Didymella pinodes over Winter and Spring Pea Cropping Seasons

“…DNA was extracted from lyophilized mycelium as described by Lodhi et al (46), quantified by measuring the optical densities of extracts at 260 and 280 nm with a NanoDrop 1000 spectrophotometer (Thermo Scientific), and adjusted to a final concentration of 100 ng · liter Ϫ1 for amplified fragment length polymorphism (AFLP) analysis. AFLP analysis was carried out as described by Vos et al (47) with modifications used by Le May et al (32). AFLP reactions were performed independently three times, using the same set of primers with reference strains and a random sample of 10 isolates from the collection, as well as independent DNA preparations of the same strains, to estimate the repeatability of fragment scoring.…”

“…This pathogen is known to present individuals differing widely in their abilities to cause disease on peas (28)(29)(30)(31)(32). In contrast to many necrotrophic fungi, D. pinodes can simultaneously develop its anamorph (asexual) and teleomorph (sexual) forms on the same plant during the growing season (32,33). Moreover, due to the indeterminate growth of the pea crop, both pycnidia (asexual) and pseudothecia (sexual) can be observed simultaneously on the same plant organs.…”

“…D. pinodes is a necrotrophic, polyphagous, polycyclic, homothallic fungus that causes ascochyta blight on winter and spring pea fields worldwide (26,27). This pathogen is known to present individuals differing widely in their abilities to cause disease on peas (28)(29)(30)(31)(32). In contrast to many necrotrophic fungi, D. pinodes can simultaneously develop its anamorph (asexual) and teleomorph (sexual) forms on the same plant during the growing season (32,33).…”

“…These features are more important for D. pinodes than for pathogens whose modes of reproduction are dissociated, since they help to increase population diversity and have an impact on pathogenicity. A recent study (32) showed that epidemics on winter and spring pea crops are due to a single D. pinodes population. Despite the long intercropping season, D. pinodes populations from winter and spring pea crops display high, but similar, genetic variability.…”