Yuma Agricultural Center, University of Arizona
This study was conducted at the Yuma Valley Agricultural Center. The soil was a silty clay loam (7-56-37 sand-silt-clay, pH 7.2, O.M. 0.7%). Lettuce was seeded, then sprinkler-irrigated to germinate seed on Nov 2, 2021 on double rows 12 in. apart on beds with 42 in. between bed centers. All other water was supplied by furrow irrigation or rainfall. Treatments were replicated five times in a randomized complete block design. Each replicate plot consisted of 25 ft of bed, which contained two 25 ft rows of lettuce. Plants were thinned Jan 6, 2021 at the 3-4 leaf stage to a 12-inch spacing. Treatment beds were separated by single nontreated beds. Treatments were applied with a tractor-mounted boom sprayer that delivered 50 gal/acre at 100 psi to flat-fan nozzles spaced 12 in apart.
Month |
Max |
Min |
Avg |
Rain |
November |
84 |
53 |
67 |
0.00 |
December |
71 |
44 |
57 |
0.27 |
January |
72 |
42 |
57 |
1.31 |
February |
75 |
43 |
59 |
0.02 |
March |
81 |
48 |
65 |
0.00 |
Downy mildew (caused by Bremia lactucae) rating was done on 3 different commercial lettuce varieties Magosa, Bobcat and 180. Disease was first seen on 1-9-22. Foliar applications were made on January 13th, January 24th, and February 1st 2022. Disease rating was done on February 21st 2022. Disease severity was determined by rating 10 plants within each of the five replicate plots per treatment using the following rating system: 0 = no downy mildew present; 0.5 = one to a few very small downy mildew colonies on bottom leaves; 1 = downy mildew present on bottom leaves of plant; 2 = downy mildew present on bottom leaves and lower wrapper leaves; 3 = downy mildew present on bottom leaves and all wrapper leaves; 4 = downy mildew present on bottom leaves, wrapper leaves, and cap leaf; 5 = downy mildew present on entire plant. These ratings were transformed to percentage of leaves infected value. This was done for each variety of lettuce, and then each variety was used as replicate for statistical analysis.
The data in the table illustrate the degree of disease control obtained by application of the various treatments in this trial. Variety 180 did not show any downy mildew disease symptoms. Most of the treatments exhibited activity against the disease to some extent on variety Magosa and Bobcat. The most effective fungicides, that held the percentage of leaves that were infected to 20% or less, included Zampro, Revus, Cevya, and Timorex ACT/Ranman for the variety Magosa and Zampro, Cevya and Timorex ACT/Ranman for the variety Bobcat. Phytotoxicity symptoms were not noted for any treatments in this trial.