Apparently powdery mildew in lettuce caused by Erysiphe cichoracearum is normally a problem of minor importance in the Central Coast growing areas. On a recent visit (late July) to a lettuce field in the west Santa Maria valley, the early signs and symptoms of powdery mildew were observed. This field will probably be harvested without receiving a treatment for powdery mildew control. However, young fields in the area may be at risk, so a review of lettuce powdery mildew may be in order.
Symptoms of this disease usually first appear on the frame leaves. Small (1/4-inch diameter) circular spots of fuzzy white or semi-clear growth appear. Initially, there is no leaf-yellowing. However, as the infection sites enlarge, circular spots under the fuzzy growth begin to yellow. The fungus growth can enlarge to cover the whole leaf. The infected tissues will curl, lose their natural luster, yellow, and die. Both powdery and downy mildews produce a white, fuzzy mycelial growth on the surface of the host. The following signs and symptoms can be used to identify which causal organism is present:
| 1. Erysiphe cichoracearum |
| 2. Fungus spores are produced in a chain on a single conidiophore. |
| 3. Lesions tend to be circular and spread across leaf veins. |
| 4. Fungus mycelium and spores are external to the host. |
| 5. Host tissues usually yellow after fungus growth is evident. |
| 1. Bremia lactucae |
| 2. Fungus spores are produced singly on a branched conidiophore. |
| 3. Lesions tend to be angular and confined within leaf veins. |
| 4. Fungus mycelium is internal to the host, and only the conidiophore and spores are external. |
| 5. Host tissues usually yellow before fungus growth is evident. |
Control of powdery mildew is problematic. Fortunately, most powdery mildew infections in lettuce occur near maturity, so harvest may be possible before treatment is necessary. If treatment is called for, wettable sulfur and several copper compounds are the only materials available which will provide some suppression of powdery mildew in lettuce.
Most drip irrigation systems are operated on regular intervals during the growing season. Irrigation frequencies vary from 1-4 days between irrigations. If you irrigate every day, in each irrigation event you need to apply enough water for crop consumptive use in that day. However, if you irrigate once every three days, you need to apply enough water in each irrigation that will last for three days. Crop water requirements (Etc ) can be determined from reference evapotranspiration (real time Eto or normal Eto) and crop coefficients (Kc). Reference evapotranspiration for a particular location can be obtained from California Irrigation Management Information System (CIMIS) weather stations. The consumptive water use for a particular crop can be estimated from:
If you do not know the crop coefficient for your crop, you can estimate it from percent canopy coverage (if you have 50% crop coverage, then the crop coefficient is approximately 0.5). Early during the season when crop coverage is very small, you need to use percent wetted area as an estimate for the crop coefficient.
Once you decide how much water you need in a particular irrigation event, you need to know the flow rate (volume of water per unit time) of your system. This can be accomplished by installing a flow meter. While most flow meters give you the flow rate in gallons per minute (gpm) or cubic feet per second (cfs), many flow meters give you total volume in acre-feet or other similar units. Almost all low volume irrigation systems have flow meters. If your system does not have one, it is a good idea to get one. To determine irrigation time for your drip irrigation system, you need to know the following:
Irrigation time (T) can be determined from one of the following equations:
where:
The above assumes 100% distribution uniformity (DU), most properly designed and managed drip irrigation systems have DU’s in the range of 85-95%. Therefore, you may need to increase the irrigation time to adjust for the uniformity of application. It is not uncommon to see DU’s as low as 50%, to maintain an efficient system, you need to check your system components on a regular basis and evaluate drip system performance at the beginning of each growing season.
Converting flow rate (gpm) to depth (in)
You need to know the average flow rate (gpm) and the area being irrigated (acres). If you have flow rate (gpm) and area being irrigated (acres), depth applied (inches) = [flow rate (gpm)*irrigation time (hours)]/[area (acres)* 452.74].
Example:
If the flow rate is 1,000 gpm, and irrigation time is four hours, and you have 50 acres:
Conversion factors:
1 acre = 43560 ft2
1 ft3 = 7.48 gallons
1 gallon = 3.785 liters
1 gpm = 60 gph
1 cfs = 449 gpm
24-hour run: 1 cfs ::: 2 Ac-ft per 24 hr.
12-hour run: 1 cfs ::: 1 Ac-ft per 12 hr.
Information from: Imperial Co. "Ag. Briefs," April 1998.