- Author: Steven T. Koike
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.
- Author: Jian Long Bi
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.
- 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
- 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. |
- Author: Steven T. Koike
- Author: Jian Long Bi
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 (1/10th 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.