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

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

The effect of temperature and moist period duration on sporulation of <i>Peronospora belbahrii</i>.

<p>Infected basil plants at 6dpi were kept in a dew chamber at 20°C in the dark. Percentage sporulaing leaf area was assessed after 9 or 16 hours of moist period. Different letters in the graphs indicate significant differences between means (Tukey-Kramer HDS analysis, α = 0.05).</p

The dynamics of sporulation of <i>Peronospora belbahrii</i> on leaf surface of intact basil plants kept in a dew chamber at 18°C in the dark.

<p>Photos were taken with the aid of a UV epi-fluorescent microscope after staining with calcofluor (3h–8h) or calcofluor and basic aniline blue (11h and 14h). Bar in 3h-8h = 100μm; Bar in 11h = 50 μm; Bar in 14h = 25 μm.</p

The effect of dew period duration on infection of basil plants with <i>Peronospora belbahrii</i>.

<p><b>A</b>- Continuous dew period of 1 to 9 hours at 15°C in the dark. <b>B</b>- The effect of 10 min dry period during 9h dew period on infection. The 9h dew period was interrupted at 1h time intervals after inoculation by 10 min of dryness, after which the plants were returned to the dew chamber. Different letters in the graphs indicate significant differences between means (Tukey-Kramer HDS analysis, α = 0.05).</p

The effect of dew period duration and spore concentration (spores/ml) on infection of basil plants with <i>Peronospora belbahrii</i> at 15°C in the dark.

<p>Bars indicate the standard deviation of the means.</p

Spore germination and penetration of <i>Peronospora belbahrii</i>.

<p><b>A</b>- Germination in water, 6h at 15°C in the dark. Bar = 100μm. Calcofluor staing. <b>B</b>- Penetration into a basil leaf at 6h at 15°C in the dark. Bar = 25μm. <b>C</b>—Penetration into a basil leaf at 24h at 15°C in the dark. Bar = 50 μm. Note in <b>B</b> the holes produced by the pathogen in the periclinal walls of the epidermal cells. Note in <b>C</b> the callose (yellow) deposits around the penetrated sites. Photos <b>B</b> and <b>C</b> were taken with the aid of Olympus A70 fluorescent microscope at different field depths after staining with calcofluor and basic aniline blue.</p

Investigation of Seed transmission in <i>Peronospora belbahrii</i> the Causal Agent of Basil Downy Mildew

Downy mildew in sweet basil (Ocimum basilicum L.) caused by the oomycete pathogen Peronospora belbahrii Thines was first recorded in Israel in 2011. Within one year, the pathogen has spread all over the country, causing devastating economic damage to basil crops. Similar outbreaks were reported in Europe, the USA, and Asia. Seed transmission and seedling trade were suggested as possible explanations for this rapid spread. Here, we show that P. belbahrii can develop systemically in artificially inoculated basil plants in growth chambers. It may reach remote un-inoculated parts of the plant including the axillary buds but not the roots or seeds. To verify whether transmission of the disease occurs via seeds, we harvested seeds from severely infected, field-grown basil plants. Harvests were done in four seasons, from several basil cultivars growing in three locations in Israel. Microscopic examinations revealed external contamination with sporangia of P. belbahrii of untreated seeds, but not of surface-sterilized seeds. Pathogen-specific PCR assays confirmed the occurrence of the pathogen in untreated seeds, but not in surface-sterilized seeds. Contaminated seeds were grown (without disinfection) in pasteurized soil in growth chambers until the four&#8211;six leaf stage. None of several thousand plants showed any symptom or sporulation of downy mildew. PCR assays conducted with several hundred plants grown from contaminated seeds proved no latent infection in plants developed from such seeds. The results confirmed that (i) P. belbahrii can spread systemically in basil plants, but does not reach their roots or seeds; (ii) sporangia of P. belbahrii may contaminate the surface, but not the internal parts, of seeds produced by infected basil plants in the field: and (iii) contaminated seeds produce healthy plants, which carry no latent infection. The data suggest that P. belbahrii in Israel is seed-borne, but not seed-transmitted

The effect of interrupted dew period on sporulation of <i>Peronospora belbahrii</i>.

<p>Infected basil plants at 6dpi were kept in a dew chamber at 20°C in the dark. After 2, 3, 4, 5, or 6 hours plants were taken out, fanned for 10 min at 30°C to dry off leaf surface moisture and returned back into the dew chamber. Percentage sporulating leaf area was estimated after 9 hours. Different letters in the graphs indicate significant differences between means (Tukey-Kramer HDS analysis, α = 0.05).</p

The effect of relative humidity on sporulation of <i>Peronospora belbahrii</i> in detached basil leaves.

<p>Leaves were kept in sealed boxed over saturated salt solutions for 24h at 20°C in the dark. Different letters on bars indicate significant differences between means (Tukey-Kramer HDS analysis, α = 0.05).</p