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.

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.

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.