Article submitted by:

Steve Koike, Plant Pathology Farm Advisor
University of California Cooperative Extension
Monterey County

Two important virus pathogens have been affecting coastal lettuce crops for a number of years. As expected, both problems have shown up again this summer of 2010. These virus diseases are familiar to experienced growers and pest control advisors. However, one should exercise caution in diagnosing these problems because their respective symptoms can resemble each other. 

Impatiens necrotic spot virus (INSV): INSV-infected plants have leaves with brown to dark brown spots and dead (necrotic) areas; this necrotic tissue can resemble burn damage caused by pesticide or fertilizer applications. Extensive necrosis can cause much of the leaf to become brown, dry, and dead. Some leaf yellowing can also be observed. Yellowing and the brown spotting tend to be observed on the newer leaves near the center of the plant’s growing point. If plants are affected with INSV early in their development, growth may be stunted. All lettuce types are susceptible, and INSV has been confirmed on iceberg, butterhead, romaine, and leaf lettuces. INSV can infect many other crops and weed species; the virus is vectored by thrips. 

Lettuce necrotic stunt virus (LNSV) and Tomato bushy stunt virus (TBSV): Diseased lettuce can be severely stunted, especially if infected early in plant development. The oldest, outer leaves can be severely yellowed. Brown, necrotic spots and lesions later develop in these outer leaves. The younger, inner leaves remain dark green in color, but can be rough and leathery. LNSV/TBSV infects only romaine, butterhead, and leaf lettuces; modern cultivars of iceberg lettuce are immune. The LNSV/TBSV virus complex is a soilborne problem. No vector (insect, nematode, fungus) is known to spread these viruses.

See table below for comparison of these virus disease symptoms. For help in diagnosing these and other plant problems, submit samples to the UC Cooperative Extension diagnostic lab in Salinas.

Symptom comparisons for INSV and LNSV/TBSV pathogens of lettuce
Symptom		INSV	LNSV/TBSV
Presence of yellowing, chlorosis		yes	yes
Yellowing mostly on older leaves		no	yes
Brown necrotic spots, lesions		yes	yes
Stunting if infected early		yes	yes
Central part of plant remains green		no	yes
Affects romaine and leaf lettuce		yes	yes
Affects iceberg head lettuce		yes	no

Impatiens necrotic spot virus (INSV) on lettuce


Lettuce necrotic stunt virus (LNSV) on lettuce.

Article submitted by:
Jian Bi, Entomology Farm Advisor
University of California Cooperative Extension
Monterey County

Leafminers (Liriomyza spp.) are important insect pests of lettuce.  Major leafminer species in the Coastal lettuce is Liriomiza langei.  Adult leafminers are small flies with yellow triangular spots between wings (Fig. 1).  These flies lay eggs inside lettuce leaves.  Leafminer adult flies puncture leaves to suck up plant sap, causing holes or bumps on the leaves, while the hatched larvae (maggots) feed inside leaves causing tunnels (mines).  Feeding damage caused adults and larvae provide entrances of disease organisms and can render the lettuce unmarketable. 

Chemical control is an important component in the integrated management of leafminers.  Due to the emergence of insecticide resistance, introduction of novel insecticides with distinct modes of action into the current system for control of leafminers is urgently needed.  We recently started a project to evaluate novel insecticides and their mixtures with other insecticides against leafminers in lettuce (Fig. 2).  These include cyazypyr (a novel chemistry that has not been registered in lettuce), Coragen, Durivo (a pre-mixture of active ingredients from Platinum and Coragen), Voliam Xpress (a pre-mixture of active ingredients from Warrior and Coragen).  Trigard and Agri-mek were applied as comparisons.  Broader spectrum of insect pest control is expected from these mixtures.  We expect to finish this trial in late August. 

 

 
Fig. 1.  Adult leafminers are small flies with yellow triangular spots between wings.

Fig 2.  We recently started a project to evaluate novel insecticides and their mixtures with other insecticides against leafminers in lettuce.

 

Article submitted by:
Michael Cahn, Irrigation and Water Resources Farm Advisor
University of California Cooperative Extension
Monterey County

I have heard growers complain that the nearest CIMIS station is too far away from their ranch to provide accurate reference evapotranspiration (ETo) estimates.   The CIMIS staff have recently improved the spatially accuracy of ETo estimates.  CIMIS ETo estimates can now incorporate solar radiation data from the Geostationary Operational Environmental Satellite (GOES).  GOES is the satellite that monitors ocean temperatures (think El Niño).   The data (Figure 1) has a spatial resolution of 2 km (1.25 mi) so local effects of cloud cover and fog can be factored into ETo estimates.  Relative humidity, air temperature, and wind speed data which are also required for the ET estimates are estimated by triangulating from stations closest to the location of interest.  The more CIMIS stations operating in your region, the more accurately CIMIS can estimate ET for your field.  Finally, like “MyCIMIS,” the spatial CIMIS reports can be emailed to you daily or weekly and in multiple file formats.

You can access spatial CIMIS ETo data from the CIMIS website (wwwcimis.water.ca.gov)

1.  Log on to MyCIMIS (you may need to set up a user account which is free).
2. Go to the spatial CIMIS tab.
3. Click on the link “Map Reports.”
4. Choose the option “Map Coordinates” to bring up Google Mapping tool (Figure 2).
5. Select the fields for which you would like to have spatial CIMIS ETo estimates  (Figure 3).
6. Select if you would want the spatial CIMIS report emailed daily or weekly or not emailed (Figure 4).
7. Select the units for the data (english/metric), start and end dates, and file format.  Note that csv format can be imported into spreadsheet programs like excel (Figure 5). 

 

Figure 1. Map of solar radiation (right) and daily reference ET estimates from GOES data for California on May 10 , 2010.  Solar radiation is expressed in units of MegaJoules/m² .  ET estimates are expressed in millimeters.

Figure 2. Selecting “map coordinate” option brings up the google map screen.

Figure 3. Fields of interest can be selected by zooming in on the Google Map screen.  Longitudes and latitudes of locations are displayed below the map. 

Figure 4. After selecting fields of interest, the user chooses email, unit, date, and format options, and submits the query.  

Point	Lat	Long	Date	CIMIS ETo (in/day)	Sol Rad (Ly/day)
1	36.82	-121.78	5/5/2010	0.16	640.93
1	36.82	-121.78	5/6/2010	0.18	637.74
1	36.82	-121.78	5/7/2010	0.17	654.02
1	36.82	-121.78	5/8/2010	0.016	650.08
1	36.82	-121.78	5/9/2010	0.13	514.43
1	36.82	-121.78	5/10/2010	0.1	353.03
1	36.82	-121.78	5/11/2010	0.15	655.33
2	36.86	-121.7	5/5/2010	0.17	661.39
2	36.86	-121.7	5/6/2010	0.19	656.88
2	36.86	-121.7	5/7/2010	0.18	662.51
2	36.86	-121.7	5/8/2010	0.16	657.43
2	36.86	-121.7	5/9/2010	0.13	481.99
2	36.86	-121.7	5/10/2010	0.09	274.04
2	36.86	-121.7	5/11/2010	0.16	671.71
3	36.93	-121.7	5/5/2010	0.17	663.62
3	36.93	-121.7	5/6/2010	0.19	668.97
3	36.93	-121.7	5/7/2010	0.18	655.7
3	36.93	-121.7	5/8/2010	0.17	662.72
3	36.93	-121.7	5/9/2010	0.12	426.91
3	36.93	-121.7	5/10/2010	0.1	304.96
3	36.93	-121.7	5/11/2010	0.16	672.15

Figure 5.  Example of Spatial ETo data imported into excel from an emailed CIMIS report. First column refers to field number.   

Article submitted by:

Steve Koike, Plant Pathology Farm Advisor
University of California Cooperative Extension
Monterey County

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.