Results of Fungicide Timing Experiment for Control of Lettuce Drop

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AG BRIEFS – FEBRUARY, 2005

Results of Fungicide Timing Experiment for Control of Lettuce Drop

Thomas Turini, Ronald Cardoza and Barry Pryor

AG BRIEFS –SEPTEMBER, 2005

Lettuce drop can be extremely damaging in low desert production areas, particularly in fields in which lettuce has been grown for several consecutive seasons. Damage caused by this disease can be as little as the loss of a few isolated plants in a large field, or it can cause the collapse of 70% of the plants in a field. The first symptom usually appears as a water-soaked lesion on the stem that develops into a soft, brown crown rot. The outer leaves wilt, turn yellow and lay flat on the soil surface. The fungus will progress inward affecting younger layers of leaves. Eventually, the entire plant collapses.

Lettuce drop is caused by two closely fungal species, Sclerotinia sclerotiorum and S. minor. Both fungi produce hard, durable sclerotia, which are composed of densely packed, melanized cells. The sclerotia of S. minor are 1/16-1/8 inch in length and S. sclerotiorum sclerotia are ¼-½ inch in length. These structures can be found in cottony mycelium associated with the disease schlerotia and are capable of surviving in the soil for many years under unfavorable conditions.

In the low desert production areas of the southwestern US, the majority of the damage caused by S. sclerotiorum is caused by eruptive germination of the sclerotia. Wet, cool soil conditions (32o - 82oF; optimum 60o - 70 oF) favor infection.

When there is a history of lettuce drop in a field, preventative fungicide applications are used for control. Common commercial practice is to treat within 3 days following thinning and a second application 10- to 14-days later. However, there have been conditions under which this treatment schedule did not provide commercially acceptable levels of control.

During the 2004-05 lettuce season, an experiment was conducted at University of California Desert Research and Extension Center to compare application timings for control of drop. On 9 November, S. sclerotiorum sclerotia were placed on the soil surface (9.3 sclerotia/ft), iceberg lettuce cv. ‘Winterhaven’ seed was sown and the field was sprinkler irrigated. On 18 November, the sprinklers were removed and the lettuce was furrow irrigated.

Rovral 4F fungicide at 2.0 pts per acre in 50 gallons water per acre was applied with a CO2-pressurized backpack sprayer at 30 psi. The spray boom equipped with four 8002VS Teejet nozzles per seed line. Nozzles were directed at base of the plants.

Application dates for each of the treatments were as follows: a. 19 Nov 04 (1 true leaf) and 17 Dec 04 (one day after thinning, at 5 to 6 true leaves), b. 19 Nov 04, c. 17 Dec 04, d. 17 Dec 04 and 1 Jan 05 (rosette stage). An untreated control was also included.

The experimental design was a randomized complete block with 3 replications. Each plot consisted of a 25 ft length of 2 beds with a 3 ft planted buffer between plots. On 16 Dec 04, 1, 14 and 25 Jan 2005, plants in each plot with drop symptoms were recorded.

Under the conditions of this study, the Rovral 4F provided the best control of drop when applied on 19 Nov with or without the 17 Dec application and when applied on 17 Dec and 1 Jan (Table 1). The 17 Dec application alone provided control as compared to the untreated control, but did not perform as well as the other timings included in this study. It is important to consider that weather conditions following the applications may have an influence on disease incidence. Soil and ambient temperatures gradually declined (Fig. 1).

AG BRIEFS –SEPTEMBER, 2005

Table 1. Affect of Rovral 4F 2.0 oz/A application timing on drop incidence on iceberg lettuce at UC Desert Research Extension Center, 2005.

z Rovral was applied in 50 gallons water per acre with a CO2-pressurized backpack sprayer at 30 psi.

y One hundred plants per plot were evaluated for obvious symptoms of drop and S. sclerotiorum sclerotia.

Fig 1. Ambient and soil (6 in depth) temperatures and rainfall during Nov, Dec 04 and Jan 05 at UC Desert Research and Extension Center. (CIMIS station 87).

Alfalfa Varieties 2005

Juan N. Guerrero

AG BRIEFS –SEPTEMBER, 2005

An alfalfa variety trial was planted at the University of California Desert Research and Extension Center (UCDREC) in 2004. At the present time, we only have one year of data. Conclusions should not be based on a single year’s data.

The last three year alfalfa variety trial at UCDREC was completed in 2003.

The results of that trial are in Table 1. Unfortunately, we do not have any local data for the Round Up® Ready alfalfa varieties that will be released soon. Forage genetics has informed us that they will not be making available any seed of their Round Up® alfalfas in the Imperial Valley this year.

AG BRIEFS –SEPTEMBER, 2005

Table 1. UC IMPERIAL VALLEY ALFALFA CULTIVAR TRIAL 2001-2003 YIELDS. TRIAL PLANTED 10/11/00.

AG BRIEFS –SEPTEMBER, 2005

Cutback Irrigation

Khaled M. Bali

AG BRIEFS –SEPTEMBER, 2005

Cutback irrigation is simply reducing the inflow rate of water to graded furrows after the water reaches the lower end of the field. The basic objective of cutback irrigation is to reduce surface runoff from the field by matching the inflow rate with the average infiltration capacity of field. Cutback irrigation is one of the irrigation management practices that are commonly used to improve the efficiency of surface irrigation systems. Cutback irrigation increases irrigation efficiency by reducing the volume of runoff water (tailwater). Cutback irrigation is similar to tailwater recovery systems where irrigation efficiency is improved by reducing runoff losses.

Application efficiency is the ratio of the amount of water that is beneficially used by the crop to the total amount applied. Beneficial use includes crop water use and leaching. When cutoff irrigation is employed, the amount of runoff water generally decreases which improves application efficiency. Runoff can be eliminated when the rate of inflow equals the average infiltration rate over the entire length of the field. It is not any easy task for an irrigator to adjust the inflow rate so it matches the average infiltration rate over the entire field. Cutback irrigation may require the installation of values at the head end of the field to control inflow rate. Infiltration rate varies with time and space (along the field). Therefore, cutback irrigation is commonly used to reduce runoff and not to eliminate tailwater.

Since infiltration rate decreases over time, cutback irrigation is usually employed by cutting inflow rate several times during irrigation events. In general, multiple cutbacks are more effective in reducing runoff than a single cutback in flow rate. However, multiple cutbacks may require the installation of automated valves. As it is the case with other water conservation measures, implementation of cutback irrigation requires extra labor and improvements in the water delivery system.

AG BRIEFS –SEPTEMBER, 2005

Insecticide Trial For Worm Pests In Lettuce, 2004.

Eric T. Natwick

AG BRIEFS –SEPTEMBER, 2005

A field trial was conducted at the University of California Desert Research and Extension Center near Holtville, CA, to look at foliar insecticide applications for efficacy against beet armyworm and cabbage looper on head lettuce, variety Jupiter. The experimental design was a randomized complete block with four replicates. Plots measured 50 ft by 13.33 ft; 4 beds per plot no 40" centers. Foliar sprays were applied at 52 gpa at 40 psi using a Spider Trac Sprayer with three TJ-60 11003VS nozzles per bed. All foliar treatments included an adjuvant (Latron CS-7) at 0.125 % (v / v). Insecticide treatments were applied on November 5 and December 2, 2004. Insecticide treatments and rates of application are listed in Table 1. Evaluations were made by counting the numbers of cabbage looper larvae and beet armyworm larvae per twenty plants in each plot on each sampling date. The insecticide BAS 320 I,

active ingredient metaflumizone, is under developed by BASF. It is in the

semicarbizone class of chemistry. BASF expects registration is late 2006 with first sales in 2007. BAS 320 I shows broad activity on lepidopterous pests.

There were no differences among the treatments for beet armyworms on November 2, three days prior to insecticide treatments. All insecticide treatments had significantly fewer beet armyworm larvae compared to the untreated control on November 9, 5 days after treatment (DAT) but there were no differences among the insecticide treatments (ANOVA, P < 0.05) (Table 1). There were no other mean separations among the treatments for beet armyworms on subsequent sampling dates. However, all insecticide treatments had significantly fewer beet armyworm larvae compared to the untreated control for seasonal means.

AG BRIEFS –SEPTEMBER, 2005

Table 1. Beet Armyworms per Twenty Lettuce Plants Following Insecticide Treatments, Holtville, CA, 2004.

Mean separations within columns by LSD (P 0.05). z PTM = Post treatment mean.

AG BRIEFS –SEPTEMBER, 2005

There were no differences among the treatments for beet cabbage loopers on November 2, three days prior to insecticide treatments. All insecticide treatments had significantly fewer cabbage looper larvae compared to the untreated control on sampling dates from November 9 through December 15 and for the seasonal means, but there were no differences among the insecticide treatments (ANOVA, P < 0.05) (Table 2).

AG BRIEFS –SEPTEMBER, 2005

Table 2. Cabbage Loopers per Twenty Lettuce Plant Following Insecticide Treatments, Holtville, CA 2004.

Mean separations within columns by LSD (P 0.05). z PTM = Post treatment mean.