Nitrate leaching from vegetable production along the Central Coast is under greater scrutiny and is the subject of proposed regulations by the Regional Water Quality Control Board (RWQCB). The regulations as written have stipulated that leachate from agricultural lands should not exceed the public health limits for nitrate in drinking water of 10 ppm nitrate-nitrogen.

To date there has been little information developed on the quantities of nitrate contained in leachate from lettuce production. In 2009 we conducted a nitrogen fertilizer trial in which we applied 10, 75, 150, 225 and 300 lbs of N/A and water was applied at 116% of evapotranspiration. In order to measure leachate from the plots, suction lysimeters were installed to a depth of 2 feet deep in the soil (photo 1). During each irrigation, suction in the lysimeters was maintained at 20-25 centibars which was assumed to be the leachable fraction of soil water.   After each irrigation, leachate was collected and analyzed for nitrate concentration.

The 10 lbs N/A was a low N treatment (and yielded substantially lower than other treatments), but even in this treatment had leachate nitrate-N concentrations substantially greater than the 10 ppm nitrate-N drinking water standard (see graph below) for the majority of the early season. The concentration of nitrate-N in this treatment declined to below the drinking water standard for the final third of the growing season. These data give us a glimpse into nitrate levels of leachate from vegetable production fields. Even treatments with little applied N can have substantial quantities of nitrate in the leachate. This indicates that monitoring of the concentration of nitrate in the leachate may not be a consistently useful tool for understanding the quantity of N leached.  

  

 

 

Figure 1: Nitrate-N concentrations in leachate over the growing season of romaine lettuce (for simplicity, we pooled the leachate nitrate levels of the highest three nitrogen fertilizer treatments 150, 225 and 300 lbs N/A, as they were not significantly different from each other)

The cucumber crop in central coast California is a minor crop, with only a modest acreage planted annually. However, a major disease threatens this commodity that is grown both out in the field and inside greenhouses. In recent years, a very aggressive, destructive strain of downy mildew (the pathogen is Pseudoperonospora cubensis) has devastated cucumber plantings. Leaves first develop angular shaped lesions that turn yellow. Later, the tissue in these lesions dies and becomes brown (photo 1). In most cases the diagnostic purple gray mycelium and spores develop on the leaf undersides (photo 2). As disease progresses, entire leaves decline and the plants collapse due to severe infection. Downy mildew also infects squash and watermelon, though this current problem is most problematic on cucumber.

California growers are hardly alone in this situation. Last year the aggressive downy mildew damaged cucumber crops in various parts of the USA, along the eastern seaboard stretching from New York down to Florida, and from there extending west as far as Wisconsin, Illinois, Missouri, Louisiana, and Texas. For California, downy mildew was reported on production cucumber in the central coast and other regions, and on seed cucumber crops in the upper San Joaquin Valley.

Management of this apparently new cucumber strain is difficult. Organic producers have few options because protectant sprays do not appear to help, and suitable resistant cultivars have not yet been identified. For conventional growers, early preventative sprays should be made (see the UC IPM website: http://www.ipm.ucdavis.edu/PMG/r116101611.html). This cucumber situation is yet another case illustrating how this group of pathogens is able to change and cause problems for growers. Central coast growers are already very familiar with the new races and aggressive outbreaks of lettuce and spinach downy mildews.

Plant Pathologist Steven Koike is monitoring the California cucumber situation and is collaborating with researchers in Michigan and North Carolina. He is interested in hearing about downy mildew outbreaks on cucumber in California (phone 831-759-7350; stkoike@ucdavis.edu).

 Photo 1: Angular lesions on cucumber caused by downy mildew.

Photo 2: Downy mildew lesions support the purple growth of the pathogen.

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).

Photo 1

 

Photo 2

Photo 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 (1/10^th inch) long) and pale in color.

Photo 4

Photo 5

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.

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.

Additional Information:

UC website:   http://www.ipm.ucdavis.edu/EXOTIC/eurograpevinemoth.html

CDFA website:  
http://www.cdfa.ca.gov/PHPPS/egvm/index.html

Figure 1. Adult female EGVM

Figure 2. Earlier stages of EGVM larvae are tan to yellow-brown, while latter stages become dark colored.