Two new species of the Peronospora belbahrii species complex, Pe. choii sp. nov. and Pe. salviae-pratensis sp. nov., and a new host for Pe. salviae-officinalis

Predicting the resistance of basil entries to downy mildew based on their genetics, pathogen race, growth stage, and environmental conditions

MAIN CONCLUSION

A model predicting the level of resistance of basil to downy mildew was developed. The model integrates plant age, genetic background, sporulation, disease intensity, pathogen races, and environmental data at an early stage of disease. These results can be used to select and develop new basil cultivars and accelerate the time needed in breeding for basil downy mildew resistance. Basil downy mildew (BDM) caused by the oomycete Peronospora belbahrii emerged as a global threat, rapidly becoming the most devastating disease of sweet basil (Ocimum basilicum) and other Ocimum spp. worldwide. Despite advancements in understanding its biology and epidemiology, and the availability of approved fungicides and management strategies, BDM remains economically destructive and an ongoing risk to basil production worldwide. Recently, the development and introduction of resistant cultivars have emerged as crucial tools in BDM management and the emergence of new BDM races creates new challenges to controlling this disease. The present study aimed to provide growers and breeders with insights into the survival capabilities of resistant basil cultivars under varying genetic backgrounds, pathogen races, growth stages, and various environmental conditions. Through a series of lab and field experiments, we evaluated the response of multiple resistant sources and their lineages to various isolates of P. belbahrii across different locations, using multiple indices to assess their resistance. Entries carrying the R genes Pb1/Pb2 exhibited complete resistance across all races, growth stages, and environmental conditions. Those harboring the R-gene Pb2 showed similar resistance levels, with minor variability due to growth stage. Responses of Pb1 plants varied with pathogen race, displaying full resistance to race 0 at all growth stages but displaying susceptibility to race 1. Plant cultivars possessing MRI resistance genes and their recombinant inbred lines (RIL's) exhibited variable responses to pathogen attacks, ranging from high tolerance to complete susceptibility. Some MRI RIL's showed high resistance similar to Pb2 entries. Pb0 cultivars and 'Eleonora' (unknown background) were susceptible to all races and growth stages in all experiments. Comprehensive analysis across all genetic backgrounds revealed a significant correlation (R = 0.73) between disease intensity (D.I) at the seedling stage under controlled conditions and D.I in adult plants under field conditions. Principal Component Analysis (PCA) across six experiments indicated that the primary components influencing disease outcomes were the accession, race, and growth stage, explaining 65%, 22%, and 7% of the variability, respectively. A prediction model based on the statistical parameters residual (%) and root-mean-square error (RMSE) demonstrated strong predictability, particularly regarding pathogen sporulation and daily disease development rates. The model predicted resistance probabilities with R2 values of 0.81, 0.91, and 0.93 at the second, third, and final disease score readings, respectively, significantly earlier (~ 14-21 days post-infection) than traditional assessments (~ 42 days). These findings demonstrate that resistance in basil entries against current pathogen races can be effectively assessed within weeks of disease onset, facilitating more timely and informed management decisions for growers and providing an important tool for plant breeders in search of improved BDM resistance.

Alien and cryptogenic fungi and oomycetes in Austria: an annotated checklist (2nd edition)

Fungal invasions can have far-reaching consequences, and despite increasing relevance, fungi are notoriously underrepresented in invasion science. Here, we present the second annotated checklist for alien and cryptogenic fungi and oomycetes in Austria. This list contains 375 taxa of which 278 are classified as established; compared to the first checklist from 2002, this amounts to an almost five-fold increase and the number of decade-wise first records is steadily rising since the mid-twentieth century. The introduction pathway is unclear for the vast majority of taxa, while the main means of spread within the country is unassisted secondary spread. Fungi were predominantly introduced from the Northern Hemisphere, especially North America and Temperate Asia. Rates of newly recorded alien fungi differ among phyla; the majority belongs to the Ascomycota, which experienced an 9.6-fold increase in numbers. Orders found most frequently are powdery mildews (Erysiphales, Ascomycota), downy mildews (Peronosporales, Oomycota), agarics (Agaricales, Basidiomycota), Mycosphaerellales (Ascomycota), rusts (Pucciniales, Basidiomycota) and Pleosporales (Ascomycota). The majority (about 80%) of the taxa are plant pathogens, while animal pathogens are few but severely affecting their native hosts. The dominance of pathogens in our checklist underlines the need of better tackling fungal invasions-especially in the light of emerging infectious diseases-and highlights potential knowledge gaps for ectomycorrhizal and saprobic alien fungi, whose invasion processes are often much more inconspicuous. Our results show that fungal invasions are a phenomenon of increasing importance, and collaborative efforts are needed for advancing the knowledge and management of this important group.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10530-022-02896-2.

Joint Action of Pb1 and Pb2 Provides Dominant Complementary Resistance Against New Races of Peronospora belbahrii (Basil Downy Mildew)

Sweet basil (Ocimum basilicum, 2n = 4x = 48) is susceptible to downy mildew caused by Peronospora belbahrii. The Pb1 gene exhibits complete resistance to the disease. However, Pb1 became prone to disease because of occurrence of new virulent races. Here, we show that Zambian accession PI 500950 (Ocimum americanum var. pilosum) is highly resistant to the new races. From an interspecies backcross between PI 500950 and the susceptible 'Sweet basil' we obtained, by embryo rescue, a population of 131 BC1F1 plants. This population segregated 73 resistant (58) and susceptible (1:1; P = 0.22) plants, suggesting that resistance is controlled by one incompletely dominant gene called Pb2. To determine whether allelic relationship exists between Pb1 and Pb2, we used two differential races: race 0, which is avirulent to both PI 500945 (Pb1) and PI 500950 (Pb2), and race 1, which is virulent to PI 500945 but avirulent to PI 500950. F1 plants obtained from '12-4-6' (BC6F3 derived from PI 500945) and '56' (BC3F3 derived from PI 500950) showed resistant superiority to both races through dominant complementary interaction. F2 plants segregated to race 0 as follows: 12:3:1 (immune/incomplete resistant/susceptible) as opposed to 9:3:4 to race 1, indicating that Pb1 and Pb2 are not alleles. Because joint action is contributed in F1 plants and in advanced [BC3F3(56) × BC6F3(12-4-6) F4] populations that carry both genes, it can be assumed that both accessions carry two unlinked genes but share a common signal transduction pathway, which leads to dominant complementation superiority of the resistance against different races of basil downy mildew.

Downy mildew of lavender caused by Peronospora belbahrii in Israel

Peronospora belbahrii is one of the most destructive downy mildew diseases that has emerged throughout the past two decades. Due to the lack of quarantine regulations and its possible seed-borne nature, it has spread globally and is now present in most areas in which basil is produced. While most obligate biotrophic, plant parasitic oomycetes are highly host-specific, there are a few that have a wider host range, e.g. Albugo candida, Bremia tulasnei, and Pseudoperonospora cubensis. Recently, it was shown that Peronospora belbahrii is able to infect Rosmarinus, Nepetia, and Micromeria in Israel in cross-infection trials, hinting an extended host range for also this pathogen. In this study, a newly occurring downy mildew pathogen on lavender was investigated with respect to its morphology and phylogeny, and it is shown that it belongs to Peronospora belbahrii as well. Thus, it seems that Peronospora belbahrii is currently extending its host range to additional members of the tribe Mentheae and Ocimeae. Therefore, it seems advisable to scrutinise all commonly used members of these tribes in order to avoid further spread of virulent genotypes.

Peronospora aquilegiicola made its way to Germany: the start of a new pandemic?

Peronospora aquilegiicola is a destructive pathogen of columbines and has wiped out most Aquilegia cultivars in several private and public gardens throughout Britain. The pathogen, which is native to East Asia was noticed in England and Wales in 2013 and quickly spread through the country, probably by infested plants or seeds. To our knowledge, the pathogen has so far not been reported from other parts of Europe. Here, we report the emergence of the pathogen in the northwest of Germany, based on morphological and phylogenetic evidence. As the pathogen was found in a garden in which no new columbines had been planted recently, we assume that the pathogen has already spread from its original point of introduction in Germany. This calls for an increased attention to the further spread of the pathogen and the eradication of infection spots to avoid the spread to naturally occurring columbines in Germany and to prevent another downy mildew from becoming a global threat, like Peronospora belbahrii and Plasmopara destructor, the downy mildews of basil and balsamines, respectively.