- Author: Steven Koike, Plant Pathology Farm Advisor
During the month of April 2011 in coastal California, a number of growers and PCAs are seeing wilting and stunting of young lettuce plants. Affected lettuce typically range in size from the 4-to-6 true leaf through rosette stages and are randomly distributed throughout the field as individually affected plants. Such plants initially fall behind in development and may appear slightly stunted. As the problem worsens, these plants will start to wilt during the day. Eventually the older leaves may turn yellow then brown, all foliage wilts, and the plant can collapse and die. Such a condition is caused by two different factors: ammonium toxicity and abrasion of the crown due to wind damage.
Ammonium toxicity: This problem results from the buildup of ammonium in soils planted with young lettuce seedlings. Ammonium toxicity occurs when soils are cool and the soil surface is sealed or compacted, resulting in slow nitrification rates. This disorder can also occur in fields with poorly drained, waterlogged soils. The use of fertilizers that contain ammonium can contribute to ammonium toxicity. When the lettuce root is injured by the ammonium buildup, the lettuce foliage will show the stunting and wilting symptoms described above.
To make a preliminary diagnosis of ammonium toxicity, examine the entire root system of the young lettuce. The central core of the taproot first turns yellow to light brown, then becomes dark brown to red in color (Photo 1). In severe cases, the central core collapses and a cavity forms throughout the length of the root (Photo 2). Lateral roots may be short, with blackened tips. In some situations the external surface of the root turns yellow or light brown and develops cracks; these symptoms mimic corky root disease. A more thorough analysis will require testing to eliminate the presence of the fungal pathogen Fusarium oxysporum f. sp. lactucae, the causal agent of Fusarium wilt. Because ammonium toxicity is associated with certain environmental and weather conditions, there are no effective management steps that can prevent this disorder.
Crown abrasion or wind-whipping: This problem is the result of physical damage to young lettuce plants. Strong winds will whip the lettuce crown and leaves back and forth, causing the crown to rub against the soil and become injured. These crowns will show characteristic pinching, drying, and collapse of tissue at the soil line of the lettuce (Photo 3), resulting in the stunting and wilting symptoms described above. In severe cases the lettuce plant can be completely girdled, resulting in the stubbing-off of the base of the plant; such plants will not develop to maturity (Photo 4). This “wind-whipping” of lettuce tends to be more severe in fields having coarser, sandier soils. Because wind-whipping is associated with certain environmental and weather conditions, there are no effective management steps that can prevent this disorder.
Not a disease: Neither ammonium toxicity nor wind-whipping is associated with any disease. While above-ground symptoms of both of these disorders may resemble damping-off diseases, lettuce in coastal California is not affected by damping-off pathogens such as Pythium or Rhizoctonia. Corky root disease usually does not result in the distinctive central core discoloration. Verticillium wilt may cause central core discoloration, but this disease only shows up on mature lettuce close to harvest and not on such young plants. The only disease that looks similar to ammonium toxicity is Fusarium wilt, which can infect young lettuce plants. However, Fusarium wilt always occurs in patches in a field and will not occur as individual plants that are randomly scattered throughout the planting.
Photo 1: Discoloration of the central core of the lettuce root is characteristic of ammonium toxicity.
Photo 2: Ammonium toxicity can cause the lettuce root to develop a central hollow cavity.
Photo 3: Wind-whipping results in a pinched, collapsed crown at the soil line of the lettuce plant.
Photo 4: Severe wind-whipping can completely girdle the lettuce crown.
- Author: Michael D Cahn
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)
- Log on to MyCIMIS (you may need to set up a user account which is free).
- Go to the spatial CIMIS tab.
- Click on the link “Map Reports.”
- Choose the option “Map Coordinates” to bring up Google Mapping tool (Figure 2).
- Select the fields for which you would like to have spatial CIMIS ETo estimates (Figure 3).
- Select if you would want the spatial CIMIS report emailed daily or weekly or not emailed (Figure 4).
- 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.
- 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: Richard Smith
- Author: Steven T. Koike
The rainy and cold spring weather in 2010 is apparently having an effect on head lettuce quality in Salinas Valley fields. Symptoms appear as very small, brown flecks and spots along the margins of young leaves (photo 1). Affected leaves are usually found deep within the head. It appears that these defects are occurring in multiple iceberg cultivars in various parts of the valley. Clearly this is a physiological disorder and superficially looks similar to russet spot; however, most of the flecks do not occur on the leaf midribs (photo 2) as would be typical for russet spot. Russet spot is caused by ethylene production and can occur in mature to over mature lettuce, especially following anaerobic conditions in the field. However, in this instance, the location of the flecking along the margin of the leaf more closely indicates tipburn.
The extensive nature of the problem (from Salinas to San Ardo) and the occurrence across varieties indicates that a large-scale factor like weather could be the cause. The heavy rain on April 5 followed by cloudy, cool weather may account for the currently wide distribution of the problem. We are conducting further investigations to more closely determine the cause of this problem.
These defects are not caused by any plant pathogen. Extensive testing has shown that bacterial leaf spot, anthracnose, or other lettuce disease is not associated with these brown flecks. Bacterial leaf spot (caused by Xanthomonas campestris pv. vitians), however, is common this year and should not be confused with this physiological disorder. Bacterial leaf spot occurs on the outer leaves and results in large, black, angular lesions (photo 3).
Photo 1: Lettuce defects in 2010 |
Photo 2: Typical russet spotting of lettuce |
Photo 3: Bacterial leaf spot of lettuce |