Cause The fungus, Golovinomyces cichoracearum (syn. Erysiphe cichoracearum) can infect lettuce as well as wild lettuces (Lactuca serriola, L. canadensis, L. pulchella, L. serriola, etc.). It has been reported on lettuce in Washington. The Golovinomyces group has been under reorganization by taxonomists recently, so there are potentially issues with reports published prior to 2020 and the fungal name used. Since the morphology of this fungus overlaps with several other species of Golovinomyces, it is possible that some reported hosts may be infected with a different fungus. Various members of the Asteraceae family (which also includes lettuce), both wild and cultivated species, have been reported prior to 2020 as hosts for this fungus. Crops reported as susceptible to G. cichoracearum include globe artichoke, chicory, endive, and Jerusalem artichoke, along with lettuce crops. Research out of California suggests that the host range may be limited to cultivated lettuce, although there has been a report that a cross between wild lettuce and a cultivated line has been infected with this fungus.
Throughout the growing season, G. cichoracearum produces colorless asexual spores (conidia) that are borne in chains. Dark-colored ascospores (sexual spores) can sometimes be produced in over-wintering structures known as chasmothecia, which appear as tiny dark specks in powdery mildew colonies on older leaves and senescing plants. Both types of spores may be blown in from infected plants/infected plant debris and an early report states that conidia can be windborne at least 120 miles. The disease can occur from spring through early autumn, and may occur year around in cold frame grows on the western side of Washington and Oregon as well as in greenhouses and indoor grow sites. The conidia tolerate a wide range of environmental conditions (23-93°F), and infections can occur when temperatures range between 50 and 81°F. Conidia germinate and initiate infection even when the relative humidity is between 50 and 75%, but 95-98% RH is optimum. After infection has occurred, the production of conidia in powdery mildew colonies in unaffected by the relative humidity. Light intensity can decrease the susceptibility of lettuce to powdery mildew; 186 lux at 55°F showed resistance to 'Great Lakes' lettuce compared to 28 to 37 lux.
Crisphead lettuce types are reported as more susceptible than butterhead types. Resistance has been reported only in the following varieties: Amanda Plus, Arctic King, Bath Cos, Big Green Cos, Big Boston, Bremex, Cindy, Clarion Corelli, Maruraj, Sabine, Salad Bowl, Soraya, Suttons A-1, Suzan, and Two Star. However, some varieties reported as resistant in the field were susceptible when grown in greenhouses.
Symptoms Older leaves are typically infected first. But the fungus can affect seedlings and colonize both surfaces of leaves as well as stems of lettuce. Small, discrete white patches develop on leaf surfaces. Later, patches coalesce until white-colored mycelium and spores cover the entire leaf. Severely affected plant tissues may turn yellow and then brown, as leaves die. Leaves can also appear deformed, plants may be stunted, and plants may die.
Cultural control
- Plant resistant varieties.
- Regularly scout plantings for powdery mildew, looking at both sides of older leaves.
- Avoid over-application of nitrogen fertilizers; use an optimum, balance fertility regime. Powdery mildew is enhaced by dense camopies.
- Regularly harvest plants and avoid letting plants grow past harvest maturity.
- Avoid continuous cropping of lettuce.
- Practice a 3-year rotation with nonsusceptible crops and manage volunteers and wild hosts.
Chemical control A protective fungicide program is considered the best control strategy. More than one application may be required. Tank mix or rotate among fungicide FRAC groups to avoid the development of fungicide resistance in G. cichoracearum populations.
- Bicarbonates are registered to control powdery mildew. Thorough coverage is essential.
- Bi-Carb Old Fashioned Fungicide at 4 teaspoons/2 gal water on 7- to 10-day intervals. May be applied up to the day before harvest. HO
- Kaligreen at 2.5 to 3 lb/A on 7- to 10-day intervals. May be applied up to the day before harvest. 4-hr reentry. O
- MilStop SP at 2 to 5 lb/A on 7- to 14-day intervals. Can be applied up to and including the day of harvest. 1-hr reentry. O
- Cevya (Group 3) at 3 to 5 fl oz/A on minimum interval of 7 days. Studies out of Arizona showed good efficacy (good disease control of powdery mildew) with this product at 5 fl oz/A. Preharvest interval is 0 days. 12-hr reentry.
- Cueva (Group M1) at 0.5 to 2 gal/100 gal water on 5- to 7-day intervals. May be applied on the day of harvest. 4-hr reentry. O
- Fontelis (Group 7) at 14 to 24 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action (non-Group 7). Studies out of Arizona showed good efficacy (good disease control of powdery mildew) with this product at 24 fl oz/A. Preharvest interval is 3 days. 12-hr reentry.
- Merivon Xemium (Group 7 + 11) at 4 to 11 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) applications before alternating to a fungicide in different FRAC groups (non-Group 7 and non-Group 11). Preharvest interval is 1 day. 12-hr reentry.
- Miravis Prime (Group 7 + 12) at 9.2 to 13.4 fl oz/A on 7- to 10-day intervals. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action. Studies out of Arizona showed good efficacy (good disease control of powdery mildew) with this product at 9.2 fl oz/A. Preharvest interval is 0 days. 12-hr reentry.
- Regalia (Group P5) at 0.5 to 1.5 quarts/A on 5- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
- Strobilurin fungicides (Group 11) are labeled for use. Do not make more than one (1) application of a Group 11 fungicide be-fore alternating to a labeled fungicide with a different mode of action.
- Cabrio EG at 12 to 16 oz/A on 7- to 14-day intervals. Preharvest interval is 0 day. 12-hr reentry.
- Quadris Flowable at 6 to 15.5 fl oz/A on 5- to 7-day intervals. May be applied the day of harvest. 4-hr reentry.
- Sulfur formulations are available and can be very effective on powdery mildew. Sulfur is fungitoxic in its Trilogy• Actinovate vapor phase and, therefore, is effective only when air temperatures promote volatilization. Sulfur volatilizes above 65°F but becomes phytotoxic above 95°F. Using it above 85°F is not recommended. Although sulfur reduces sporulation of established infections, it is primarily a protectant and must be applied before infection.
- Thiolux (80% sulfur) at 4 to 6 lb/A at 10- to 14-day intervals. 24-hr reentry.
Biological control Efficacy unknown in the Pacific Northwest.
- Actinovate AG at 3 to 12 oz/A as a foliar spray on 7- to 14-day intervals. 4-hr reentry. O
- Actinovate Lawn & Garden at 1/2 to 1 teaspoon/gal water. H O
- Double Nickel LC at 0.5 to 4.5 pints/A on 3- to 10-day intervals. Can be applied the day of harvest. 4-hr reentry. O
- Ecoswing at 1.5 to 2 pints/A. Preharvest interval is 0 days. 4-hr reentry. O
- Romeo at 0.45 to 0.68 lb/A on 7- to 10-day intervals starting prior to infection. Preharvest interval is 0 days. 4-hr reentry. O
References Glawe, D., and Duffield, E.C.S. 2007. First report of powdery mildew of Lactuca sativa (garden lettuce) caused by Golovinomyces cichoracearum in the Pacific Northwest. Pacific Northwest Fungi 2(5):1-4.
Koike, S.T., and Saenz, G.S. 1996. Occurrence of powdery mildew, caused by Erysiphe cichoracearum, on endive and radicchio in California. Plant Disease 80:1080.
Koike, S.T., Gladders, P., and Paulus, A.O. 2007. Vegetable Diseases: A Color Handbook. San Diego, CA, USA: Academic Press
Lebeda, A., and Mieslerova, B. 2011. Taxonomy, distribution and biology of lettuce powdery mildew (Golovinomyces cichoracearum sensu stricto). Plant Pathology 60:400-415.
Matheron, M., and Porchas, M. 2019. Assessment of fungicides for managing powdery mildew of lettuce, 2019. Plant Disease Management Reports 13:V144.
Singh, N., Porchas, M. Furr, J., and Poudel-Ward, B. 2023. Evaluation of fungicides for management of powdery mildew on lettuce, Yuma, AZ, 2022. Plant Disease Management Reports 17:V076.