Variation in pathogenicity and subsequent production of sclerotia of Sclerotinia sclerotiorum isolates in different cover crops, flower strips and weeds.

Cover crops and flower strips are used in agricultural fields as part of integrated pest management strategies. However, their potential as secondary hosts of soilborne pathogens such as Sclerotinia sclerotiorum in oilseed rape cultivation is not fully comprehended. In the current study, we evaluated the effect of pathogen virulence on the development of Sclerotinia stem/leaf rot and sclerotia production in 33 plant species from 11 botanical families using two S. sclerotiorum isolates. Furthermore, the effect of sclerotial size on carpogenic germination was studied. Results showed that the pathogen's virulence significantly affected the occurrence and development of Sclerotinia stem/leaf rot and the subsequent production of sclerotia. Among all plant species tested, 26 were more susceptible to the aggressive S. sclerotiorum isolate, which produced more and bigger sclerotia in 17 species than the less aggressive isolate. Moreover, a stronger positive correlation was found between the relative lesion length of plants inoculated with the aggressive isolate and the number of sclerotia produced by this isolate (rs = 0.572; p = 0.004). Additionally, we found that larger and heavier sclerotia produced stipes and apothecia earlier and at a greater rate than smaller ones. The sclerotia in the heavy-weight class had the highest carpogenic germination rate (82.4%), followed by the average (67.2%) and lightweight classes (59.5%). Our findings highlight the need for further investigation into the potential risks associated with cover crops, weeds, and flower strips as secondary hosts of soilborne pathogens in agricultural fields.

Effect of Pathogen Virulence on Pathogenicity, Host Range, and Reproduction of Plasmodiophora brassicae, the Causal Agent of Clubroot Disease.

A series of greenhouse experiments was conducted to evaluate the effect of Plasmodiophora brassicae virulence on clubroot development and propagation of resting spores in 86 plant species from 19 botanical families. Plants were artificially inoculated with two isolates of P. brassicae, which were virulent on clubroot-resistant oilseed rape cultivar Mendel [pathotype 1; P1 (+)] or avirulent on this cultivar (P1). Clubroot severity and the number of resting spores inside the roots were assessed 35 days post inoculation. Typical clubroot symptoms were observed only in the Brassicaceae family. P1 (+)-inoculated species exhibited more severe symptoms (two- to 10-fold more severe), bigger galls (1.1- to 5.8-fold heavier), and greater numbers of resting spores than the P1-inoculated plants. Among all Brassica species, Bunias orientalis, Coronopus squamatus, and Raphanus sativus were fully resistant against both isolates, whereas Camelina sativa, Capsella bursa-pastoris, Coincya monensis, Descurainia sophia, Diplotaxis muralis, Erucastrum gallicum, Neslia paniculata, Sinapis alba, Sinapis arvensis, Sisymbrium altissimum, Sisymbrium loeselii, and Thlaspi arvense were highly susceptible. Conringia orientalis, Diplotaxis tenuifolia, Hirschfeldia incana, Iberis amara, Lepidium campestre, and N. paniculata were completely or partially resistant to P1 isolate but highly susceptible to P1 (+). These results suggest that the basis for resistance in these species may be similar to that found in some commercial cultivars, and that these species could contribute to the buildup of inoculum of virulent pathotypes. Furthermore, the pathogen DNA was detected in Alopecurus myosuroides, Phacelia tanacetifolia, Papaver rhoeas, and Pisum sativum. It can be concluded that the number and diversity of hosts for P. brassicae are greater than previously reported.