- Author: Steven T. Koike
- Author: Richard Smith
In the months of July and August of 2010, a number of vegetable crops in California’s central coast were affected by calcium deficiency disorders. Usually referred to as “tipburn” (except in the case of celery, for which the disorder is called “blackheart”), these problems are often triggered by environmental factors and hence can affect a large number of acres and plantings during windows of time when tipburn is occurring in Salinas and other coastal valleys.
Tipburn symptoms on lettuce, endive, radicchio, and spinach are distictive. Symptoms occur on the margins of developing leaf tips and initially consist of light to dark brown lesions and necrosis. In romaine, tipburn is often first seen on the small veins along the margin of young leaves. In severe cases tipburn can progress and result in extensive damage to these leaf margins. Symptomatic leaves are usually found within the inner whorls of open head vegetables and underneath the enclosing wrapper leaves of closed head vegetable types. For spinach, tipburn always affects the inner, newly developing leaves which may develop a hooded appearance as the leaf continue to expand around the dead tissue. For blackheart of celery, symptoms form on the margins of developing leaf tips deep within the central growing point. Such symptoms consist of light to dark brown speckling, lesions, and necrosis. As the celery grows, the damaged tissues turn black and the affected foliage grows up and out of the inner plant whorl.
For tipburn of artichoke, the margins of immature leaves turn black as in celery. In addition, the immature artichoke flower buds develop black lesions along the upper tips and edges of flower bracts. Cauliflower also develops tipburn, with the inner wrapper leaves enclosing the cauliflower head turning tan to light brown.
Tipburn is a localized calcium deficiency. It often develops along the margins of leaves in the final weeks before harvest when the plant growth rate is at its highest. It is often unrelated to soil calcium levels. Because calcium is not very mobile within the plant, expanding tissues on young leaves and growing points may run short of this essential element and begin to develop deficiency symptoms. Conditions that favor rapid plant growth (warm temperatures and high fertilization rates) or low transpiration rates (foggy conditions) may create conditions that trigger this disorder.
Managing this disorder is difficult. Varieties vary in susceptibility, but tipburn resistance is only available in head lettuce. Maintaining even soil moisture levels and preventing the root zone from drying out helps increase calcium uptake and may reduce the risk or severity of tipburn. For field grown vegetables, soil and foliar calcium supplements have not provided consistent control.
For further information on tipburn see Monterey County Crop Notes - July/August 2007
Initial symptoms of tipburn on romaine occur on veins at the edge of the leaf.
Tipburn of butter lettuce.
Tipburn of cauliflower.
Blackheart of celery.
Tipburn of artichoke.
- Author: Richard Smith
In 2008 we received a 24C for use of Dual Magnum on spinach; however, two issues make the current version of the label difficult to use: 1) the plant back interval for lettuce is 12 months which seriously limits its utility in Salinas Valley rotations, and 2) the preharvest interval (PHI) is 50 days. The plant back issue is still not resolved; however progress was recently made on the PHI.
The IR4 program conducted residue studies to change the PHI to 21 days. We just received news about the residue trials: Dual Magnum residues collected at this interval exceeded the current tolerance and a 21 day PHI will not be possible. As a result, Syngenta is currently trying to settle for a PHI of 40 days. This would be an improvement over the current PHI of 50 days, but still leaves Dual Magnum in a grey zone for use on clipped spinach which commonly matures in 30 days or less.
One alternative that we have explored is the application of Dual Magnum prior to planting. The question that comes up is do you lose a certain percentage of the Dual Magnum if it is applied to the top of the bed and it sits for a period of time before being incorporated into the soil with sprinkler irrigation following planting. In San Ardo in 2009 we conducted a trial in which Dual Magnum was applied to shaped beds on July 1 and the field was planted on July 21. We observed good efficacy at the 0.75 and 1.0 pint rates (Table 1). There was little efficacy at the 0.50 pint/A rate when the Dual Magnum remained on the soil surface for 21 days prior to planting; normally we see reasonable weed control at the 0.50 pint/A rate when Dual Magnum is applied immediately following planting and incorporated into the soil with sprinkler irrigation. It therefore appears that Dual Magnum can remain on the soil surface for at least 20 days, but higher rates may need to be used to obtain weed control equivalent to what is needed for at-planting applications. We will need further evaluations of this application technique to better understand the rates and timing.
Table 1. Weed counts and phytotoxicity rating on August 6, 2009
Treatment |
Material |
Lbs a.i./A |
Purslane |
Malva |
Other weeds |
Total weeds |
Phyto |
Dual Magnum |
0.50 pint |
0.48 |
41.3 |
0.8 |
1.3 |
43.3 |
0.0 |
Dual Magnum |
0.75 pint |
0.72 |
4.0 |
1.8 |
2.3 |
8.0 |
0.8 |
Dual Magnum |
1.00 pint |
0.96 |
1.0 |
2.8 |
4.3 |
8.0 |
1.3 |
Untreated |
---- |
---- |
3.0 |
11.8 |
21.8 |
36.5 |
0.0 |
Pr>Treat |
|
|
<0.001 |
<0.001 |
<0.001 |
0.002 |
0.005 |
LSD 0.05 |
|
|
16.4 |
3.7 |
5.7 |
17.0 |
0.7 |
- 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