Replacing Mancozeb with Alternative Fungicides for the Control of Late Blight in Potato.

Mancozeb (MZ) is a broadly used fungicide for the control of plant diseases, including late blight in potatoes caused by the oomycete Phytophthora infestans (Mont.) De Bary. MZ has been banned for agricultural use by the European Union as of January 2022 due to its hazards to humans and the environment. In a search for replacement fungicides, twenty-seven registered anti-oomycete fungicidal preparations were evaluated for their ability to mitigate the threat of this disease. Fourteen fungicides provided good control (≥75%) of late blight in potted potato and tomato plants in growth chambers. However, in Tunnel Experiment 1, only three fungicides provided effective control of P. infestans in potatoes: Cyazofamid (Ranman, a QiI inhibitor), Mandipropamid (Revus, a CAA inhibitor), and Oxathiapiprolin + Benthiavalicarb (Zorvek Endavia, an OSBP inhibitor + CAA inhibitor). In Tunnel Experiment 2, these three fungicides were applied at the recommended doses at 7-, 9-, and 21-day intervals, respectively, totaling 6, 4, and 2 sprays during the season. At 39 days post-inoculation (dpi), control efficacy increased in the following order: Zorvec Endavia > Ranman > Revus > Mancozeb. Two sprays of Zorvec Endavia were significantly more effective in controlling the blight than six sprays of Ranman or four sprays of Revus. We, therefore, recommend using these three fungicides as replacements for mancozeb for the control of late blight in potatoes. A spray program that alternates between these three fungicides may be effective in controlling the disease and also in avoiding the build-up of resistance in P. infestans to mandipropamid and oxathiapiprolin.

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.

Transfer of Downy Mildew Resistance from Wild Basil (Ocimum americanum) to Sweet Basil (O. basilicum).

Sweet basil (Ocimum basilicum) is susceptible to downy mildew caused by the oomycete foliar pathogen Peronospora belbahrii. No resistant varieties of sweet basil are commercially available. Here, we report on the transfer of resistance gene Pb1 from the highly resistant tetraploid wild basil O. americanum var. americanum (PI 500945, 2n = 4x = 48) to the tetraploid susceptible O. basilicum 'Sweet basil' (2n = 4x = 48). F1 progeny plants derived from the interspecific hybridization PI 500945 × Sweet basil were resistant, indicating that the gene controlling resistance (Pb1) is dominant, but sterile due to the genetic distance between the parents. Despite their sterility, F1 plants were pollinated with the susceptible parent and 115 first backcross generation to the susceptible parent (BCs1) embryos were rescued in vitro. The emerging BCs1 plants segregated, upon inoculation, 5:1 resistant/susceptible, suggesting that resistance in F1 was controlled by a pair of dominant genes (Pb1A and Pb1A'). Thirty-one partially fertile BCs1 plants were self-pollinated to obtain BCs1-F2 or were backcrossed to Sweet basil to obtain the second backcross generation to the susceptible parent (BCs2). In total, 1 BCs1-F2 and 22 BCs2 progenies were obtained. The BCs1-F2 progeny segregated 35:1 resistant/susceptible, as expected from a tetraploid parent with two dominant resistant genes. The 22 BCs2 progenies segregated 1:1 resistant/susceptible (for a BCs1 parent that carried one dominant gene for resistance) or 5:1 (for a BCs1 parent that carried two dominant genes for resistance) at a ratio of 4:1. The data suggest that a pair of dominant genes (Pb1A and Pb1A') residing on a two homeologous chromosomes is responsible for resistance of PI 500945 against P. belbahrii.

Epidemiology of Basil Downy Mildew.

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.

Nocturnal Fanning Suppresses Downy Mildew Epidemics in Sweet Basil.

Downy mildew is currently the most serious disease of sweet basil around the world. The oomycete causal agent Peronospora belbahrii requires ≥ 4h free leaf moisture for infection and ≥7.5h of water-saturated atmosphere (relative humidity RH≥95%) at night for sporulation. We show here that continued nocturnal fanning (wind speed of 0.4-1.5 m/s) from 8pm to 8am dramatically suppressed downy mildew development. In three experiments conducted during 2015, percent infected leaves in regular (non-fanned) net-houses reached a mean of 89.9, 94.3 and 96.0% compared to1.2, 1.7 and 0.5% in adjacent fanned net-houses, respectively. Nocturnal fanning reduced the number of hours per night with RH≥95% thus shortened the dew periods below the threshold required for infection or sporulation. In experiments A, B and C, the number of nights with ≥4h of RH≥95% was 28, 10 and 17 in the non-fanned net-houses compared to 5, 0 and 5 in the fanned net-houses, respectively. In the third experiment leaf wetness sensors were installed. Dew formation was strongly suppressed in the fanned net-house as compared to the non-fanned net-house. Healthy potted plants became infected and sporulated a week later if placed one night in the non-fanned house whereas healthy plants placed during that night in the fanned house remained healthy. Infected potted basil plants sporulated heavily after one night of incubation in the non-fanned house whereas almost no sporulation occurred in similar plants incubated that night in the fanned house. The data suggest that nocturnal fanning is highly effective in suppressing downy mildew epidemics in sweet basil. Fanning prevented the within-canopy RH from reaching saturation, reduced dew deposition on the leaves, and hence prevented both infection and sporulation of P. belbahrii.

Inheritance of Resistance to Powdery Mildew Race 1W in Watermelon.

Powdery mildew caused by Podosphaera xanthii is a major disease of watermelon in Israel. In this study, 291 accessions of Citrullus spp. were evaluated for resistance against P. xanthii race 1W. Only eight accessions exhibited high level of resistance. Inheritance of resistance against P. xanthii race 1W was studied by crossing three resistant accession of Citrullus lanatus var. citroides BIU 119, PI 189225, or PI 482312 with the susceptible cultivar 'Malali' or 'Sugar Baby'. Parents, F1, F2, and back cross progenies were evaluated for resistance in growth chambers at the cotyledon stage and the 4-leaf stage and in the field, at the 15-leaf stage. Resistance at the cotyledon stage was controlled by a single, partially dominant gene, whereas at the 4-leaf stage or the 15-leaf stage resistance was controlled by three complimentary, partially dominant genes. Crosses made among these resistant accessions revealed that BIU 119 and PI 189225 carry the same genes for resistance, whereas PI 482312 shares two out of three genes with both BIU 119 and PI 189225. A breeding line with high resistance level and good fruit qualities was developed from BIU 119 × HA5500.

Resistance Against Basil Downy Mildew in Ocimum Species.

Downy mildew, caused by the oomycete Peronospora belbahrii, is a devastating disease of sweet basil. In this study, 113 accessions of Ocimum species (83 Plant Introduction entries and 30 commercial entries) were tested for resistance against downy mildew at the seedling stage in growth chambers, and during three seasons, in the field. Most entries belonging to O. basilicum were highly susceptible whereas most entries belonging to O. americanum, O. kilimanadascharicum, O. gratissimum, O. campechianum, or O. tenuiflorum were highly resistant at both the seedling stage and the field. Twenty-seven highly resistant individual plants were each crossed with the susceptible sweet basil 'Peri', and the F1 progeny plants were examined for disease resistance. The F1 plants of two crosses were highly resistant, F1 plants of 24 crosses were moderately resistant, and F1 plants of one cross were susceptible, suggesting full, partial, or no dominance of the resistance gene(s), respectively. These data confirm the feasibility of producing downy mildew-resistant cultivars of sweet basil by crossing with wild Ocimum species.

Light suppresses sporulation and epidemics of Peronospora belbahrii.

Peronospora belbahrii is a biotrophic oomycete attacking sweet basil. It propagates asexually by producing spores on dichotomously branched sporophores emerging from leaf stomata. Sporulation occurs when infected plants are incubated for at least 7.5h in the dark in moisture-saturated atmosphere at 10-27°C. Exposure to light suppresses spore formation but allows sporophores to emerge from stomata. Incandescent or CW fluorescent light of 3.5 or 6 µmoles.m(2).s(-1) respectively, caused 100% inhibition of spore formation on lower leaf surface even when only the upper leaf surface was exposed to light. The inhibitory effect of light failed to translocate from an illuminated part of a leaf to a shaded part of the same leaf. Inhibition of sporulation by light was temperature-dependent. Light was fully inhibitory at 15-27°C but not at 10°C, suggesting that enzyme(s) activity and/or photoreceptor protein re-arrangement induced by light occur at ≥15°C. DCMU or paraquat could not abolish light inhibition, indicating that photosystem I and photosystem II are not involved. Narrow band led illumination showed that red light (λmax 625 nm) was most inhibitory and blue light (λmax 440 nm) was least inhibitory, suggesting that inhibition in P. belbahrii, unlike other oomycetes, operates via a red light photoreceptor. Nocturnal illumination of basil in the field (4-10 µmoles.m(2).s(-1) from 7pm to 7am) suppressed sporulation of P. belbahrii and reduced epidemics of downy mildew, thus reducing the need for fungicide applications. This is the first report on red light inhibition of sporulation in oomycetes and on the practical application of light for disease control in the field.