Efficacy of Fungicides for Management of
Powdery Mildew on Lettuce in 2006
Michael E. Matheron and Martin Porchas
Abstract
Powdery mildew on lettuce is caused by the fungus Golovinomyces
cichoracearum (Erysiphe cichoracearum). The disease is favored by moderate
to warm temperatures and relatively dry weather conditions. Several fungicides
were evaluated for their ability to suppress development of powdery mildew on
lettuce in 2006. Powdery mildew was first detected Jan 26 in this trial. The
data in the accompanying table illustrate the degree of control obtained by
applications of the various materials tested in this trial. Among treatments, the
degree of powdery mildew suppression ranged from virtually complete to
minimal; however, all treatments significantly reduced the severity of both
mildew diseases compared to nontreated plants. This trial was initiated as a
combined downy and powdery mildew trial; therefore, some of the products
were placed in the test specifically for downy mildew. Due to low humidity
levels and no rainfall during the trial, no downy mildew developed.
Introduction
Powdery mildew, caused by the fungus Golovinomyces cichoracearum (formerly known as Erysiphe
cichoracearum), can develop rapidly in spring lettuce during March and April in western Arizona, as the crop nears
maturity, when moderate to warm temperatures and dry environmental conditions prevail. The first signs of disease
can occur as early as December or January. Successful chemical control of powdery mildew requires the presence of
an effective fungicide on plants before disease onset. Successive applications of fungicides are required to maintain
disease control until harvest. This field trial was initiated to test the efficacy of some new fungicides not registered
for use on lettuce and compare these chemistries to existing products, combinations of materials, and rotational
treatment programs for management of powdery mildew.
Materials and Methods
This study was conducted at the Yuma Valley Agricultural Center. The soil was a silty clay loam (7-56-37 sand-siltclay, pH 7.2, O.M. 0.7%). Lettuce ‘Winterhaven’ was seeded Nov 14, 2005 on double rows 12 in. apart on beds
with 40 in. between bed centers, then germinated with sprinkler irrigation from Nov 17 to 22. Additional sprinkler
irrigations were performed Dec 1 and 14, followed by furrow irrigations Jan 13, Jan 30, Feb 16 and Feb 28.
Treatments were replicated five times in a randomized complete block design. Each replicate consisted of 25 ft of
bed, which contained two 25 ft rows of lettuce. Plants were thinned Dec 28 at the 3-4 leaf stage to a 12 in. spacing.
Treatment beds were separated by single nontreated beds. Treatments were applied with a tractor-mounted boom
This is part of the University of Arizona College of Agriculture 2006 Vegetable Report, index at:
http://cals.arizona.edu/pubs/crops/az1419/
sprayer (nozzles spaced 12 in. apart) that delivered 50 gal/acre at 100 psi. Foliar applications of treatments were
made Jan 26, Feb 10, Feb 27 and Mar 6, 2006. Maximum and minimum ranges (°F) of air temperature were as
follows: Dec, 2005, 59-82, 32-49; Jan, 2006, 62-80, 30-52; Feb, 66-83, 33-62; Mar 1 to 15, 55-76, 38-55. Maximum
and minimum ranges (%) for relative humidity were as follows: Dec 2005, 39-96, 10-37; Jan 2006, 22-92, 9-41; Feb,
19-86, 5-30; Mar 1 to 15, 66-96, 11-59. No rainfall occurred during this trial. Downy mildew, caused by Bremia
lactucae, did not develop during the course of this trial; however, small colonies of powdery mildew, caused by
Golovinomyces cichoracearum, were first detected 23 Jan, 2006. The severity of powdery mildew was determined
Mar 13 to 15 by rating 10 plants in the middle of each of the five replicate plots per treatment using the following
rating system: 0 = no powdery mildew present; 1 = powdery mildew present on bottom leaves of plant; 2 = powdery
mildew present on bottom leaves and lower wrapper leaves; 3 = powdery mildew present on bottom leaves and all
wrapper leaves; 4 = powdery mildew present on bottom leaves, wrapper leaves and cap leaf; 5 = powdery mildew
present on entire head. For powdery mildew, yield loss due to rejected lettuce heads would normally begin to occur
on plants with a rating above 2.0.
Results and Discussion
The data in the following table illustrate the degree of control obtained by applications of the various materials tested
in this trial. Among treatments, the degree of powdery mildew control ranged from virtually complete to minimal;
however, all treatments significantly reduced disease severity compared to nontreated plants. This trial was initiated
as a combined downy and powdery mildew trial; therefore, some of the products were placed in the test specifically
for downy mildew. All of these downy mildew chemistries, such as Aliette, Forum, Maneb, Ranman, Reason,
Ridomil Gold, and V-10161, provided some suppressive effect on powdery mildew. Due to low humidity levels and
no rainfall during the trial, no downy mildew developed. Some leaf burn was noted on plants treated with Aliette;
however, phytotoxicity symptoms were not noted for any other treatments.
Fungicide resistance management, which seeks to minimize the risk of a plant pathogen population becoming
resistant to one or more fungicides, is imperative for the preservation of fungicide effectiveness. Resistance
management is achieved by applying mixtures of fungicides or alternating among different classes of chemistries to
prevent or minimize a shift in the pathogen population toward tolerance or insensitivity to one or more disease
control compounds. The availability of more chemistries with different modes of action could help in efforts to
control powdery mildew of lettuce and at the same time support fungicide resistance management efforts for plant
disease control chemicals of importance for this crop.
2005-2006 Powdery Mildew of Lettuce Fungicide Trial
Mike Matheron and Martin Porchas, The University of Arizona, Yuma Agricultural Center, Yuma, AZ
Treatment
Rate of product per acre
Treatment
Disease
dates 1
rating 2
Untreated control
Procure 480SC
Procure 480SC
Alt. with Microthiol Disperss 80WDG
Maneb 75DF + Microthiol Disperss 80DF
Microthiol Disperss 80DF
Ridomil Gold EC
Quintec 250SC
Procure 480SC
Quadris 2.08SC
V-10161 4FL + V-10118 0.41EC
V-10161 4FL + V-10118 0.41EC
V-10118 0.41EC
V-10118 0.41EC
Forum 500SC + Maneb 75D + Penetrator
Ranman 400SC + Prophyt + Silwet L-77
Ranman 400SC + Silwet L-77
Ridomil Gold + Maneb 75DF
Alt. with Quadris 2.08SC
V-10118 0.41EC
Forum 500SC + Cabrio 20EG + Penetrator
Prophyt
V-10161 4FL + Dyne-amic
Ranman 400SC + Maneb 75DF + Silwet L-77
Ranman 400SC + Prophyt + Silwet L-77
Maneb 75DF
Ranman 400SC + Prophyt + Silwet L-77
V-10161 4F + Ridomil Gold EC
Quadris 2.08SC
Reason (500 g/l) + Bond
Alt. with Maneb 75DF
Aliette 80WDG
Ranman 400SC + Prophyt + Silwet L-77
Kinetic
V-10161 4F + Aliette 80WDG
Table continued on next page
------8.0 fl oz
8.0 fl oz
10.0 lb
2.0 lb + 10.0 lb
10.0 lb
4.0 fl oz
6.0 fl oz
8.0 fl oz
15.4 fl oz
2.9 fl oz + 6.2 fl oz
2.9 fl oz + 12.5 fl oz
6.2 fl oz
12.5 fl oz
6.0 fl oz + 2.0 lb + 12.0 fl oz
2.75 fl oz + 1.0 qt + 2.0 fl oz
2.75 fl oz + 2.0 fl oz
2.0 fl oz + 1.6 lb
15.4 fl oz
9.3 fl oz
6.0 fl oz + 0.75 lb + 6.0 fl oz
2.0 qt
2.9 fl oz + 8.0 fl oz
2.75 fl oz + 1.5 lb + 2.0 fl oz
2.1 fl oz + 1.0 qt + 2.0 fl oz
2.0 lb
2.75 fl oz + 2.0 qt + 2.0 fl oz
2.9 fl oz + 4.0 fl oz
15.4 fl oz
8.2 fl oz + 16.0 fl oz
2.0 lb
5.0 lb
2.1 fl oz + 2.0 qt + 2.0 fl oz
4.0 fl oz
2.9 fl oz + 2.0 lb
------1,2,3,4
1,3
2,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,3
2,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,3
2,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,3
2,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
3.1
0
0
0
0
0.1
0.4
0.4
0.6
0.7
0.8
0.8
0.9
0.9
0.9
0.9
1.0
1.1
1.1
1.1
1.1
1.2
1.2
1.4
1.4
1.4
1.5
1.5
1.6
1.6
1.6
Treatment
Rate of product per acre
Treatment
dates 1
Disease
rating 2
Continued from preceding page
Forum 500SC + Maneb 75DF + Penetrator
Alt. with Maneb 75DF
Alt. with Forum + Cabrio 20WG + Penetrator
Forum 500SC + Maneb 75DF + Penetrator
Alt. with Maneb 75DF
Alt. with Forum + Cabrio 20WG + Penetrator
Dyne-amic
V-10161 4FL + Kinetic
Silwet L-77
V-10161 4F + Maneb 75DF
Previcur Flex
Alt. with Maneb 75DF
BmJ WP
V-10161 4FL
Silmatrix + Kinetic
Actinovate SP + Kinetic
Silmatrix + Kinetic
6.0 fl oz + 1.6 lb + 12.0 fl oz
2.0 lb
6.0 fl oz + 1.0 lb + 12.0 fl oz
6.0 fl oz + 1.6 lb + 12.0 fl oz
2.0 lb
6.0 fl oz + 1.0 lb + 12.0 fl oz
8.0 fl oz
2.9 fl oz + 4.0 fl oz
+ 2.0 fl oz
2.9 fl oz + 1.6 lb
2.0 pt
2.0 lb
0.14 lb
2.9 fl oz
2.0 qt + 4.0 fl oz
6.0 oz + 4.0 fl oz
1.0 qt + 4.0 fl oz
LSD (Least Significant Statistical Difference, P=0.05)
1
2
3
1,2
3
4
1,2
3
4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,3
2,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1,2,3,4
1.7
1.7
1.8
2.0
2.0
2.0
2.2
2.2
2.2
2.4
2.5
2.6
0.13
Treatment dates: 1 = Jan 26; 2 = Feb 10; 3 = Feb 27; 4 = Mar 6, 2006.
Disease ratings were performed Mar 13 to 15. The severity of powdery mildew was determined by using
the following rating system:
0 = No powdery mildew colonies present on plant.
1 = Powdery mildew present on bottom leaves.
2 = Powdery mildew present on bottom leaves and lower wrapper leaves.
3 = Powdery mildew present on bottom leaves and all wrapper leaves.
4 = Powdery mildew present on bottom leaves, wrapper leaves, and cap leaf.
5 = Powdery mildew present on entire head.
Yield loss due to rejected lettuce heads would normally begin to occur on plants with a rating greater than
2.0. Small powdery mildew colonies (2 to 3 mm in diameter) were first observed on some plants Jan 23.
Least Significant Statistical Difference at P = 0.05.