Basil downy mildew (BDM) caused by the oomycete Peronospora belbahrii is a destructive disease of sweet basil (Ocimum basilicum) worldwide. It originated in Uganda in the 1930s and recently spread to Europe, the Middle East, Americas, and the Far East. Seed transmission may be responsible for its quick global spread. The pathogen attacks leaf blades, producing chlorotic lesions with ample dark asexual spores on the lower leaf surface. Oospores may form in the mesophyll of infected leaves. The asexual spores germinate on a wet leaf surface within 2 h and penetrate into the epidermis within 4 h. Spore germination and infection occur at a wide range of temperatures from 5 to 28.5°C. Infection intensity depends on the length of dew period, leaf temperature, and inoculum dose. The duration of latent period (from infection to sporulation) extends from 5 to 10 days, depending on temperature and light regime. The shortest is 5 days at 25°C under continuous light. Sporulation requires high humidity but not free leaf wetness. Sporulation occurs at 10 to 26°C. At the optimum temperature of 18°C, the process of sporulation requires 7.5 h at relative humidity ≥ 85%, with 3 h for sporophores emergence from stomata and 4.5 h for spore formation. Sporophores can emerge under light or darkness, but spore formation occurs in the dark only. Limited data are available on spore dispersal. Spores dispersed from sporulating plants contaminate healthy plants within 2 h of exposure. Settled spores may survive on leaf surface of healthy plants for prolonged periods, depending on temperature. Seed transmission of the disease occurs in Europe, but not in Israel or the United States. P. belbahrii in Israel also attacks species belonging to Rosemarinus, Nepeta, Agastache, Micromeria, and Salvia but not Plectranthus (coleus). A Peronospora species that infects coleus does not infect sweet basil. Control of BDM includes chemical, physical, and genetic means. The fungicide mefenoxam was highly effective in controlling the disease but resistant populations were quickly selected for in Israel and Europe rendering it ineffective. A new compound oxathiapiprolin (OSBP inhibitor) is highly effective. Nocturnal illumination of basil crops controls the disease by preventing sporulation. Daytime solar heating suppressed the disease effectively by reducing spore and mycelium viability. The most effective physical means is fanning. Nocturnal fanning prevents or limits dew deposition on leaf surfaces, and as a result, infection and sporulation diminish and epidemics are prevented. Genetic resistance occurs in wild basil and its transfer to sweet basil is under way.

Six wild accessions of Cucumis sativum were evaluated for resistance against each of the 23 isolates of the downy mildew oomycete Pseudoperonospora cubensis. The isolates originated from Israel, Europe, USA, and Asia. C. sativum PI 197088 (India) and PI 330628 (Pakistan) exhibited the highest level of resistance against multiple isolates of P. cubensis. Resistance was manifested as reduced lesion number, lesion size, sporangiophores and sporangia per lesion and enhanced encasement of haustoria with callose and intensive accumulation of lignin in lesions of both Plant Introductions (PIs) compared to the susceptible C. sativum SMR-18. In the field, much smaller AUDPC (Area Under Disease Progress Curve) values were recorded in PI 197088 or PI 330628 as compared to SMR-18. Each PI was crossed with SMR-18 and offspring progeny plants were exposed to inoculation with each of several isolates of P. cubensis in growth chambers and the field during six growing seasons. F1 plants showed partial resistance. F2 plants showed multiple phenotypes ranging from highly susceptible (S) to highly resistant (R, no symptoms) including moderately resistant (MR) phenotypes. The segregation ratio between phenotypes in growth chambers ranged from 3:1 to 1:15, depending on the isolate used for inoculation, suggesting that the number of genes, dominant, partially dominant, or recessive are responsible for resistance. In the field, the segregation ratio of 1:15, 1:14:1, or 1:9:6 was observed. F2 progeny plants of the cross between the two resistant PI’s were resistant, except for a few plants that were partially susceptible, suggesting that some of the resistance genes in PI 197088 and PI 330328 are not allelic.

No abstract

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.