- Author: Steven T. Koike
- Author: Jian Long Bi
The spring growing conditions have been responsible for several problems that affected head lettuce in coastal California. Rain and cold temperatures have allowed significant development of bacterial leaf spot (Xanthomonas campestris pv. vitians) and a physiological disorder possibly related to tipburn. In addition, recent samples, received by UC Cooperative Extension, have been infested by an insect. Field personnel and pest control advisors have also been detecting this problem.
Underneath the wrapper leaves, inner layers are being fed upon by the larval stage (maggot) of a fly insect. Damage consists of holes and breaks in the leaves where the maggot has been feeding (photos 1, 2, 3).
Edges of the damaged areas turn tan to brown. Such feeding damage can occur anywhere along the leaf and midrib tissue. Careful examination of the inner leaves will likely reveal the presence of the maggot (photo 4) and/or the pupa (photo 5). Maggots are small (approximately 7 mm (1/10th inch) long) and pale in color.
The insect damage, which consists of actual holes in the tissue, is distinct from the physiological problem that typically does not result in breaks in the leaf and which is usually restricted to the leaf margins. The bacterial leaf spot disease affects mostly outer leaves and results in characteristically black lesions.
Identification of the fly is pending and Entomology Farm Advisor Jianlong Bi will be investigating this further.
- Author: Larry J Bettiga
European grapevine moth (EGVM), Lobesia botrana, was detected in several Napa County vineyards in the fall of 2009. Native to Mediterranean Europe this invasive insect’s preferred host is grape. Although it is related to other tortricid moths found in vineyards (orange tortrix and omnivorous leafroller) it does not tie leaves together or feed on leaf tissue. EGVM larvae feed on grape flower parts and berries. Late season feeding on berries results in increased incidence of bunch rots.
The adult moth is approximately ¼ inch long with the first pair of wings having a mosaic-pattern (fig.1). Eggs are laid singly which is different than the overlapping egg masses of other vineyard tortricid moths. There are five larval instars. The fully-grown fifth instar is approximately ½ inch (fig. 2). Mature larvae spin a cocoon in which they pupate.
In response to the Napa County finds a statewide trapping program was started in March 2010 to determine if EGVM exists in other grape growing areas of California. The program is a coordinated effort between the county agricultural commissioners, CDFA and USDA. Specific pheromone lures in red “Delta” sticky traps are being used. Early results from the 2010 trapping program has expanded the quarantine area in Napa County and moths have been caught in traps in Sonoma, Solano, Mendocino and Fresno Counties.
Growers wanting to conduct their own trapping programs can purchase the traps and the EGVM specific pheromone lure from commercial vendors.
Figure 1. Adult female EGVM
|Figure 2. Earlier stages of EGVM larvae are tan to yellow-brown, while latter stages become dark colored.|
- Author: Steven T. Koike
Downy mildew of lettuce, caused by Bremia lactucae, is the very common foliar disease that results in the familiar yellow to brown leaf lesions and accompanying white sporulation on the lesions. However, the systemic phase of lettuce downy mildew may be less familiar to growers and pest control advisors. In the spring of 2009, systemic downy mildew was very common in coastal California. Currently in 2010, systemic downy mildew is not as serious but is still being observed in some coastal plantings.
Symptoms of systemic downy mildew may be seen on both lettuce leaves and the central, internal core of the lettuce plant. For leaf symptoms, examine the plant for large, elongated regions of the leaf that are discolored and turning dark green to brown. Such regions often develop along the midrib of the leaf and extend into the flat, outer leaf panels (photos 1, 2). White sporulation is often not present on these infected areas until late in disease development. Note that for many systemically infected lettuce plants, these leaf symptoms are absent and the only evident symptoms are in the internal core.
To check for systemic infections in the plant core, cut open and examine the central part of the plant; these tissues will show a dark brown to black streaking and discoloration (photos 3, 4). In some cases, systemically infected plants may be slightly stunted or late in maturing. Exercise caution, however, before concluding that internal core discoloration is due only to systemic downy mildew. Other important lettuce problems (Verticillium wilt, Fusarium wilt, ammonium toxicity) can cause similar internal discolorations.
Confirmation of systemic downy mildew requires laboratory testing. Affected tissues can treated with biological stains and then examined using a microscope. Such procedures can show the presence of the characteristically thick mycelium that lacks cell cross walls (photo 5). In addition, incubating pieces of affected lettuce tissue can result in sporulation of the pathogen (photo 6, showing systemic downy mildew of cauliflower), again enabling confirmation of systemic downy mildew.
Systemic downy mildew of lettuce has not been studied extensively, so researchers do not know exactly what triggers this less common phase of the disease. Some suggest that early infection of young plants may allow the pathogen to infect the inner foliage of lettuce, resulting in pathogen access to the plant growing point. Field personnel also report that some lettuce cultivars are more severely affected than others.
|Photo 1: Brown discoloration due to systemic downy mildew infection in a lettuce leaf|
|Photo 2: Brown discoloration due to systemic downy mildew infection in a lettuce leaf.|
|Photo 3: Internal discoloration of lettuce core due to systemic downy mildew infection|
|Photo 4: Internal discoloration of lettuce core due to systemic downy mildew infection.|
|Photo 5: Blue-stained mycelium of downy mildew that has systemically infected lettuce tissues.|
|Photo 6: Sporulating downy mildew from a systemically infected piece of cauliflower stem.|
- Author: Michael D Cahn
Access to weather data from the California Irrigation Management and Information System (CIMIS) has become easier than ever due to improvements in the website (www.cimis.water.ca.gov). CIMIS is managed by the CA Department of Water Resources.
CIMIS is a network of more than 120 weather stations that operate through out the agricultural regions of California. Currently, 13 stations are located on the central coast (Figure 1). All stations record relative humidity, air temperature, wind speed and direction, and solar radiation, and are located above a standard crop of grass or alfalfa, which are referred to as reference crops. Using these weather data and a mathematical model (Penman-Monteith) , potential crop water use, also called evapotranspiration (ETo), of the reference crop can be estimated. A crop coefficient is used to adjust the reference evapotranspiration data to evapotranspiration estimates for other crops, such lettuce, strawberry, or celery. CIMIS weather stations also monitor precipitation and soil temperature, and the stations calculate dew point, net radiation, and vapor pressure from the collected data.
The CIMIS staff has made many improvements in managing the weather data over the years. They use computer algorithms to check for outlying values which are flagged in reports. They have incorporated google maps to help you locate CIMIS stations near your fields (Figure 1), and they have incorporated Satellite weather data to help improve the spatial resolution of CIMIS evapotranspiration estimates.
Perhaps most importantly, the CIMIS staff has simplified getting access to the data. You can have the data emailed to you at a specified interval and also you can specify the format of the data (excel, web report, etc):
- Go to the CIMIS website and sign up for a user ID and password on the My CIMIS tab. (There is no cost for signing up and CIMIS does not send you annoying email solicitations).
- After logging on at the My CIMIS tab, select the station(s) from which you would like to receive weather data. Add the stations to a list.
- Under My Custom Report select “customize” to create a report. Choose the file format, station list, weather parameters, and time period (1 week, 2 week etc) that the report should cover (Figure 2). You can check the box to schedule the report to be automatically emailed to you (Figure 3). Note that the CSV format can be imported into Excel. The web report can be viewed directly from your web browser.
Figure 1. Location of currently operating CIMIS weather stations on the central coast as viewed from station location map under the Spatial CIMIS tab. Station numbers are displayed in the white rectangles.
Figure 2. User can select weather data and file format on the Custom report screen of My CIMIS tab.
Figure 3. User can select to have report emailed on the Custom report screen of My CIMIS tab.
Lygus bugs (Lygus species) damage strawberry fruit by puncturing individual seeds. This, in turn, stops development of the berry in the area surrounding the feeding site causing fruit distortion called “cat-facing”. Even at moderate densities, Lygus bugs cause economic loss to strawberry growers. Lygus bugs feed on many host plant species. In the Central Coast and Santa Maria Valley, they feed on strawberries and many flowering weed species and alternate crop hosts such as mustards, pepper weed, wild radish, vetch, alfalfa, and fava beans. The adult bugs usually overwinter in these weed species while some overwinter on second-year berries when present. They start to migrate to fall plantings in the spring, but only the adults can fly from one host to another. Therefore, an understanding of Lygus bug ecology and developmental biology on strawberries and the alternative hosts will help develop effective management strategies.
Pesticides remain the primary tool for suppression of Lygus populations. Due to the emergence of pesticide resistance, it is essential to better time the few pesticides registered to control this pest. The sprays must be timed to kill the youngest immatures because the registered pesticides are less effective against the adults. This will become even more critical as IGRs and other newer products become registered that have activity against more specific life stages of Lygus.
Monitoring to detect Lygus bugs on strawberries and the alternative hosts is the first step towards successful management of this pest. The rate of Lygus bug development is directly related to the amount of heat the bugs are exposed to, so measuring the amount of heat accumulation over time can be used to tell when different developmental stages in the Lygus bug life cycle will occur. A degree-day model was developed to measure the amount of heat accumulation over the season and is an effective tool to predict the Lygus bug development, but this method has not been widely adopted by strawberry growers and their PCAs.
The specific objectives in this project are: (1) to monitor the population dynamics and developmental biology of Lygus bugs in the Central Coast and Santa Maria Valley, (2) to identify the migration pattern of Lygus bugs to/from strawberries in the Central Coast and Santa Maria Valley, (3) to establish biofix dates for the Lygus bug degree-day model at multiple sites, and calculate degree-days throughout the sampling season, and (4) to disseminate timely information to the strawberry growers and PCAs to improve their Lygus bug management decisions.
Seasonal Lygus bug life cycles are determined by systematically sampling strawberry fields and nearby flowering weed species starting early February 2010 to determine age structure (number of adults, small nymphs – 1st – 3rd instars, and large nymphs 4th – 5th instars) of the Lygus population on each host. We are currently sampling four sites in the Central Coast and two sites in the Santa Maria Valley. These sites cover a variety of climate.
Sampling in the strawberries is being done using a beating tray. The sampling unit is 10 plants that have been “beaten” to dislodge any Lygus bug present onto the tray on each sampling date. A suction sampling machine could be substituted in practice. Five areas in each field are sampled in this manner. Weeds are sampled by a sweep net, using 10 sweeps through the foliage or flowers as a sample unit and at least 5 units are sampled to determine number and age structure of Lygus bug present. Weeds that are flowering or have seeds present are preferred.
Ambient temperatures at sampling sites are recorded at hourly intervals during the sampling season using micro data loggers (HOBO temperature recorders, Onset Computer Corporation, Bourne, MA). The recorded temperature data are collected weekly for the degree-day calculation. Biofix for the degree-days is the first adult captured in strawberry plantings, and first nymph on weeds or other alternative hosts. These data are used to validate and demonstrate the Lygus bug degree-day model.
The resulting data are entered at the UC IPM Pest Monitoring web site and the web site is updated frequently. The web site address is http://www.ipm.ucdavis.edu/PM/. The username is LDDmem and the password is Membugs.
Monitoring Location information:
- Boronda Road, Salinas
- Blackie Road, Castroville
- Old Stage Road, Salinas
- San Juan Road, Pajara
- Mahoney Road, Santa Maria
- Foxen Canyon Road, Santa Maria