Control of Bremia lactucae in Field-Grown Lettuce by DL-3-Amino-n-Butanoic Acid (BABA)

DL-3-amino-n-butanoic acid (BABA) was effective in controlling downy mildew incited by Bremia lactucae Regel in lettuce plants. The two isomers of BABA, DL-2-amino-n-butanoic acid and 4-amino-butanoic acid and its s-enantiomer were ineffective compares to BABA, while the r-enantiomer was more effective. The SAR compound NaSA and its functional analogue BTH (Bion) were also ineffective compared to BABA. In growth chambers, BABA was effective when applied as a foliar spray or as a soil drench. Effective control of the disease was apparent when BABA was applied up to 5 days before inoculation or 3 days after inoculation. A foliar spray of 125 mg/L reduced disease by 50% and full control of the disease was achieved with 500 mg/L. A soil drench with 1.25 mg /pot was required for >90% control the disease. In the field, 2-4 sprays with 1g/L BABA reduced disease severity by 90% as compared to control untreated plants. BABA had no adverse effect on sporangial germination of Bremia lactucae in vitro, germination on plant leaf surface or, fungal penetration into the host. However, it prevented the colonization of the host with the pathogen.

Root treatment with oxathiapiprolin, benthiavalicarb or their mixture provides prolonged systemic protection against oomycete foliar pathogens.

Oxathiapiprolin is a fungicide effective against downy mildews of cucumber (Pseudoperonospora cubensis) and basil (Peronospora belbahrii) and late blight of tomato (Phytophthora infestans). To avoid fungicide resistance, it is recommended to apply oxathiapiprolin as a mixture with a partner fungicide that have a different mode of action. Here it is shown that a single application of oxathiapiprolin, benthiavalicarb, or their mixture (3+7, w/w) to the root of nursery plants grown in multi-cell trays provided prolonged systemic protection against late blight and downy mildews in growth chambers and in field tests. Soil application of 1mg active ingredient per plant provided durable protection of up to four weeks in tomato against late blight, cucumber against downy mildew and basil against downy mildew. Not only did the mixture of oxathiapiprolin and benthiavalicarb provide excellent systemic control of these diseases but also mutual protection against resistance towards both oxathiapiprolin and benthiavalicarb

The Novel Oomycide Oxathiapiprolin Inhibits All Stages in the Asexual Life Cycle of Pseudoperonospora cubensis - Causal Agent of Cucurbit Downy Mildew.

Oxathiapiprolin is a new oomycide (piperidinyl thiazole isoxazoline class) discovered by DuPont which controls diseases caused by oomycete plant pathogens. It binds in the oxysterol-binding protein domain of Oomycetes. Growth chambers studies with detached leaves and potted plants showed remarkable activity of oxathiapiprolin against Pseudoperonospora cubensis in cucurbits. The compound affected all stages in the asexual life cycle of the pathogen. It inhibited zoospore release, cystospore germination, lesion formation, lesion expansion, sporangiophore development and sporangial production. When applied to the foliage as a preventive spray no lesions developed due to inhibition of zoospore release and cystospore germination, and when applied curatively, at one or two days after inoculation, small restricted lesions developed but no sporulation occurred. When applied later to mature lesions, sporulation was strongly inhibited. Oxathiapiprolin suppressed sporulation of P. cubensis in naturally-infected leaves. It exhibited trans-laminar activity, translocated acropetaly from older to younger leaves, and moved from the root system to the foliage. Seed coating was highly effective in protecting the developed cucumber plants against downy mildew. UV microscopy observations made with cucumber leaves infected with P. cubensis revealed that inhibition of mycelium growth and sporulation induced by oxathiapiprolin was associated with callose encasement of the haustoria

Translaminar control efficacy of oxathiapirolin against <i>P</i>. <i>cubensis</i> in detached cucumber leaves.

<p>The compound was sprayed at a concentration of 0.0001 mg/l on the lower or the upper leaf surface and leaves were inoculated with <i>P</i>. <i>cubensis</i> on the opposite leaf surface. Disease records were taken at 14 dpi. <b>A</b>—Lesions per leaf (out of 15 droplets inoculated). <b>B</b>—Lesion size, mm. Different letters on bars indicate on significant differences at α = 0.05 (Tukey’s-Kramer HSD-test).</p

Translocation of oxathiapirolin from a treated organ to leaves.

<p><b>A</b>—Leaf 1 (in 1-leaf plants) was treated by spray. <b>B</b>—Leaf 1 (in 1-leaf plants) was treated with 10 droplets of 25 μl each. <b>C</b>—hypocotyl of 2-leaf plants was treated by spray. <b>D</b>—Soil drench in 2 leaf plants. In <b>A</b> and <b>B</b> plants were inoculated after 4 days when leaf 2 has expanded. In <b>C</b> and <b>D</b> plants were inoculate one day after treatment. Photographs were taken at 7dpi.</p

The inhibitory effect of oxathiapiprolin on development of downy mildew in cucumber depends on time of application.

<p>Detached leaves were inoculated on lower leaf surface with sporangial suspension (25 droplets/leaf) and oxathiapiprolin was applied as a spray at 0, 1, 2 or 3dpi. <b>A</b>—No lesions developed when oxathiapiprolin was applied at 0 dpi. Lesions appeared when oxathiapiprolin was applied at 1 or 2dpi, but became smaller as its dose increased. <b>B</b>—Oxathiapiprolin was applied at 3dpi. Note no effect on lesion size. Photos were taken at 6 dpi.</p

The inhibitory effect of oxathiapiprolin on cystospore germination of <i>P</i>. <i>cubensis</i>.

<p><b>A-D</b><i>In vitro</i>; <b>E-H</b><i>in vivo</i>. <b>A, C</b>—Fluorescent micrographs (with or without bright light) showing cystospore germination in water. <b>B</b>, <b>D</b>—No cystospores germination occurred in oxathiapiprolin of 0.0001 mg/l. Bars in <b>A, B</b> = 50 μm; in <b>C, D</b> = 10 μm. <b>E</b>—Scanning electron micrographs showing a germinating cystospore a control leaf. <b>F</b>—inhibitory effect of oxathiapiprolin on cystospore germination on a treated leaf. <b>G</b>—Fluorescent micrographs showing extensive mycelial growth in a control leaf at 1 dpi. Bar = 100 μm. <b>H</b>—Oxathiapiprolin inhibits zoospore discharge on a cucumber leaf surface at 1 dpi. No mycelial growth is seen. Bar = 50 μm.</p

Preventive and curative effects of oxathiapiprolin on lesion development, lesion expansion and sporagiphore formation of <i>P</i>. <i>cubensis</i> in detached leaves of cucumber.

<p>See legend to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0140015#pone.0140015.g004" target="_blank">Fig 4</a> for experimental design. <b>A</b>—Lesion size, as measured at 10 dpi. <b>B</b>—Number of sporangiophores produced at 10 dpi per 1 mm² leaf tissue as counted with the aid of a UV epifluorescent microscope. Different letters on bars indicate on significant differences at α = 0.05 (Tukey’s-Kramer HSD-test).</p

The effect of oxathiapiprolin on sporulation of <i>P</i>. <i>cubensis</i> in cucumber leaves naturally-infected with downy mildew.

<p><b>A</b>—In leaves collected from Net-house 1 on April 27, 2015 and May 1, 2015. <b>B</b>—In leaves collected from Net-house 3 on June 9, 2015 and June 14, 2015. Different letters on bars indicate on a significant difference at α = 0.05 (Tukey’s-Kramer HDS test). <b>C</b>—Close up photo showing heavy sporulation in a control leaf. <b>D</b>—Close up photo showing no sporulation in a leaf treated with oxathiapirolin of 0.0001 mg/l on June 9. Photos were taken on June 10, 2015.</p

Data from: Daytime solar heating controls downy mildew Peronospora belbahrii in sweet basil

The biotrophic oomycete Peronospora belbahrii causes a devastating downy mildew disease in sweet basil. Due to the lack of resistant cultivars current control measures rely heavily on fungicides. However, resistance to fungicides and strict regulation on their deployment greatly restrict their use. Here we report on a ‘green’ method to control this disease. Growth chamber studies showed that P. belbahrii could hardly withstand exposure to high temperatures; exposure of spores, infected leaves, or infected plants to 35-45°C for 6-9 hours suppressed its survival. Therefore, daytime solar heating was employed in the field to control the downy mildew disease it causes in basil. Covering growth houses of sweet basil already infected with downy mildew with transparent infra-red-impermeable, transparent polyethylene sheets raised the daily maximal temperature during sunny hours by 11-22°C reaching 40-58°C (greenhouse effect). Such coverage, applied for a few hours during 1-3 consecutive days, had a detrimental effect on the survival of P. belbahrii: killing the pathogen and/or suppressing disease progress while enhancing growth of the host basil plants