BASIL DOWNY MILDEW: PRESENT AND FUTURE PROSPECTS FOR CONTROL

Gene McAvoy1, Christian Miller2 & R. N. Raid3

1University of Florida, IFAS, Hendry County Extension, LaBelle, FL 2University of Florida, IFAS, Palm Beach County Extension, West Palm Beach, FL

3University of Florida, IFAS, Everglades Research & Education Center, Belle Glade, FL

Introduction

Basil downy mildew, incited by the fungal pathogen Peronospora belbahrii, was first reported in the U.S. in Florida during 2007. Since then, it has been reported in nearly every state and country where basil is grown. Affected foliar areas are frequently delineated by veination and the resulting chlorosis may be mistaken as a nutrient deficiency during the early stages of the disease. However, a light grey to brown fuzzy appearance on the underside of the leaf soon develops under favorable conditions (high humidity or leaf wetness) and is diagnostic of the disease. This fungal growth represents the sporulation, where millions of spores (conidia) are produced on small branched structures called conidiophores. Spores are easily dislodged and can spread for miles on even the slightest breezes. During later stages of the disease, leaves may turn brown and necrotic, rendering the crop totally unmarketable. The disease may also progress following harvest, causing massive losses after shipment when latent infections develop on infected tissues en route. Thus far, all sweet basil types have demonstrated a high susceptibility to the downy mildew, although a few of the specialty basils, such as lime basil, cinnamon basil, and spice basil have shown some resistance. A very aggressive disease, sweet basil growers must use a very intensive spray program to keep the disease from reaching economic levels. These experiments were conducted to investigate fungicidal efficacy and to improve prospects for control .

Figure 1. Basil downy mildew symptoms. (A) Foliar chlorosis evident from the top of the leaf. Note delineation by the leaf veins. (B) Chlorosis and necrosis evident with sever downy mildew infestations.

Materials and Methods Numerous field trials were conducted to evaluate fungicides for control of downy mildew. A composite list of fungicides, rates per acre, and active ingredients used is listed below. Experimental units consisted of four 12-ft row sections and these were arranged in randomized complete blocks with 4 replications. Soil drench treatments were applied by applying a one-time concentrated 4-inch band over emerged seedlings (Fig. 2) at the first true leaf stage in a volume of 144 GPA. Foliar sprays were applied using a CO2 backpack sprayer equipped with a hand-held boom, with three TeeJet 11003 flat-fan nozzles per two rows of basil. Foliar applications were at 7-day intervals. Disease was assessed by measuring the percentage of plants displaying visual downy mildew symptoms (incidence) and by visually estimating the percentage of the entire canopy affected by downy mildew (severity).

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100 30-Jan

6-Feb

% Incidence % Severity

Foliar (rate/a) No Drench Drench No Drench Drench

UTC 100 a 65 a 93 a 25 a

ProPhyt 3 pt 78 b 43 b 58 b 20 a

Revus 8 fl oz 3 g 1 ef 0 d 0 c

Ranman 2.75 fl 10 f 7 ef 3 d 2 c

Previcur 2pt 100 a 30 c 17 c 10 b

Presidio 4 fl 25 c 10 de 3 d 2 c

V-10208 10 fl 100 a 40 b 20 c 10 b

Zampro 14 fl 0 g 0 f 0 d 0 c

Forum 6.2 fl 15 e 5 ef 1 d 1 c

Reason 8 fl 0 g 0 f 0 d 0 c

Quadris 15.2 fl 20 d 15 d 1 d 1 c

Fungicide Active Ingred. Rate prod/A Manufacturer

Aliette WDG Fosetyl-Al 2.0 lb Bayer

Forum SC Dimethorph 6.2 fl oz BASF

Presidio Fluopicolide 4.0 fl oz Valent

Previcur SL Propamicarb 2.0 pt Bayer

ProPhyt SL Potassium Phosphite 1.0, 2.0, 3.0 pt Helena

Quadris SC Azoxystrobin 15.2 fl oz Syngenta

Ranman SC Cyazofamid 2.75 fl oz FMC

Reason SC Fenamidone 8.0 fl oz Bayer

Regalia Anthrocyanate 1% vol/vol Marrone

Revus SC Mandipropamid 8.0 fl oz Syngenta

Ridomil Gold EC Mefenoxam 8.0, 16.0, 24.0 fl oz Syngenta

V-10208 Experimental 10.0 fl oz Valent

Vacciplant Laminarin 14.0 fl oz Agrimar

Zampro Dimethomorph + ametoctradin

14.0 fl oz BASF

Figure 2. (A) Photo showing concentrated soil drench band on top of raised basil bed. Inset shows crop stage at application. (B) Downy mildew severity (%) resulting from soil drench treatments three and four weeks following application. No foliar sprays.

Table 2. Downy mildew and severity resulting following three foliar applications of the Indicated fungicide used in the absence or presence of a 16.0 fl oz soil drench with Ridomil Gold.

Results and Discussion

A comparison of compounds used as a seedling drench indicated significant difference in mildew efficacy (Fig. 2B). Presidio, Quadris, and Ridomil provided significantly longer control than the phosphites ProPhyt and Aliette, and the SAR compounds Regalia and Vacciplant, with Ridomil providing superior control at both the 8 and 16 fl oz rates. In comparing foliar sprays in the absence of a seedling drench with Ridomil (Fig. 3), Revus, Reason, and Zampro provided for the most effective reductions in DM incidence , followed by Ranman, Forum, Quadris, Presidio, and ProPhyt, in that order numerically, with all of these being aided by the Ridomil drench. With regards to DM severity, the Ridomil drench was extremely critical to providing the level of control needed when less efficacious compounds were utilized. At the present time, Ridomil Gold is labeled only as a soil treatment with a 21-day PHI. Foliar applications cannot and should not be made to avoid development of Ridomil insensitivity. Such an occurrence has already been reported in Israel. Revus, Ranman, and Quadris now have full federal labels and may form the basis for a very effective foliar rotation, particularly when used with the Ridomil drench. Although not displayed here, results from other tests have demonstrated that the addition of a phosphite, such as ProPhyt, to these foliar fungicide may further improve efficacy and help in the preservation of these valuable management tools.

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