- Author: Steven T. Koike
For much of spring and early summer 2010, coastal California continued to receive rain. Such rain significantly increased bacterial leaf spot disease problems on cilantro. Presently, bacterial leaf spot is the only foliar disease affecting cilantro in California. Bacterial leaf spot is greatly increased by splashing water from rains and overhead sprinkler irrigation. The disease initially causes water-soaked, vein-delimited spots on leaves. These spots rapidly turn dark brown in color, remain angular in shape, and can be seen from both top and bottom sides of leaves. If disease is severe, the foliage can take on a blighted appearance when leaf spots coalesce. Cilantro crops with significant amounts of this disease will be unmarketable.
The pathogen is Pseudomonas syringae pv. coriandricola. This pathovar is host specific to cilantro and does not infect celery, parsley, or other apiaceae plants. The most critical disease cycle factor is that this pathogen is seedborne in cilantro. Therefore, infested seed is the primary means by which the pathogen gets into the cilantro production system.
Because most cilantro plantings are irrigated with overhead sprinklers, disease control is very difficult. If possible, use seed that has been tested and found to be pathogen-free or that has been treated. Minimize the use of sprinklers or schedule irrigations to enhance drying of the foliage. For both conventional and organic cilantro, the only foliar treatment available is a copper fungicide. Such copper sprays provide some protection but are generally not effective enough to achieve sufficient control.
We encourage growers and pest control advisors to continue to submit cilantro samples to our UC Cooperative Extension diagnostic lab in Salinas for confirmation of this problem. While bacterial leaf spot is the only documented foliar disease of cilantro in California, other parts of the world have reported a fungus leaf spot disease on cilantro; therefore, laboratory testing of California cilantro is advisable.
Bacterial leaf spot of cilantro is recognized by its dark brown, angular leaf spots that are visible from both top and bottom sides of the leaf.
- Author: Richard Smith
Nitrogen use to grow vegetables along the Central Coast is the subject of proposed regulations by the Regional Water Quality Control Board (RWQCB). As a result there is increased interest in improving the efficiency of applied nitrogen (N) fertilizer. Understanding the uptake pattern of lettuce is critical to careful N management. Lettuce typically takes up 120-140 lbs of nitrogen in the above ground biomass with the higher uptake levels occurring on 80 inch beds with 5-6 seedlines. The uptake patterns of romaine and head lettuces are similar. Figure 1 shows the uptake of N in a lettuce crop over the growing season. At thinning (app. 31 days after planting) the crop had taken up less than 10 lbs N/A. The biggest increase in nitrogen uptake by the crop occurred between 39 days and 52 days after planting. The most difficult task in managing nitrogen fertilization of lettuce is to supply adequate levels of nitrogen during this period of high demand by the crop, but to not leach it during irrigations.
In second crop lettuce it is possible to take advantage of residual soil nitrate and use it in place of applied fertilizer. Figure 2 shows a typical pattern of nitrate-N levels in Salinas Valley soils. The data indicates that typically by the time you get to the second crop, levels of residual nitrate-N can increase above 20 ppm (which is equivalent to 80 lbs N/A) due to left over N applied to the prior crop and mineralization of nitrate-N from crop residue. Whatever the source of the residual nitrate-N, it is a substantial quantity that can be utilized by a growing lettuce crop.
Residual soil nitrate can be monitored by use of the soil nitrate quick test (see photo 1). The test provides information on levels of soil nitrate-N and can help you to decide whether you need to apply fertilizer as planned or if you can reduce This test is most effective if done at thinning to determine residual soil nitrate levels for the first post thinning fertilization and can be done again prior to subsequent fertilizer applications if desired.
Figure 1. Nitrogen uptake by lettuce over the growing season.
Figure 2. Nitrate-N in soil over the course of the growing season January to December – mean of six fields
Photo 1. Soil nitrate quick test
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
- 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.
- 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.|