SPRING DISEASES OF LETTUCE

One disease that appeared in many fields was anthracnose. This fungus disease, caused by Microdochium panattonianum, was first evident as small, yellowish, water-soaked spots on lower leaves. These spots darken, and may enlarge to 1/4-inch circles. The tissue then dries up, and leaf blade lesions may drop out, causing a shot-hole appearance. On veins and midribs, the lesions become sunken, oval, and tan-colored with a lighter beige center. These lesions are usually on the outer, lower leaves. During the spring of 2001 many heads of romaine and leaf lettuce were observed with healthy outer leaves, but with lesions on the lower portion of leaves inside the head. Many heads that appeared sound on initial examination were found to have inner diseased leaves, which necessitated stripping down the heads or abandoning the field.

At present, Quadris^TM fungicide is approved for use on lettuce. However, application must be made preventively as prophylactic applications will not prevent lesion formation and head loss in leaf and romaine-type lettuces.

Another disease that caused more head loss than usual was varnish spot. This disease is caused by Pseudomonas cichorii. Unfortunately, symptoms are often not evident until near harvest when leaves several layers into the head are found to have dark brown to near black, shiny, firm necrotic spots. The necrotic tissues may be small freckled areas or large blotches. The tissues do not break down until they are invaded by secondary soft-rot bacteria at which time a slimy, dark rot occurs within the head. Little is known about the epidemiology of this disease. Varnish spot is most often associated with sprinkler irrigation or rain events, and it is suspected that infection occurs in the late rosette or early heading stage of growth.

Varnish spot can usually be prevented by not sprinkling head lettuce after thinning. No other disease management strategies are known at this time.

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NITROGEN MANAGEMENT

Tim Hartz, Vegetable Crops Extension Specialist at UC Davis

Broccoli and cauliflower are among the most heavily fertilized of coastal vegetable crops. With fertilizer costs going up, and fertilization practices coming under serious scrutiny due to environmental concerns, it may be time to reevaluate your current fertilization practices for these crops.

Brassica crops are heavy feeders, on average taking up approximately 200-220 lbs. N per acre by first harvest. Average Salinas Valley N application levels for broccoli and cauliflower are somewhat higher than these amounts, but not radically so. It is tempting to think that fertilization rates and crop requirements are in rough balance, but that is not really so, for several reasons: a) fertilizer is only one source of available N for crop uptake, and b) only about 1/3 of total N uptake is removed from the field in the harvested product; the rest is returned to the soil in the form of crop residue.

From a whole-farm nutrient balance standpoint, current levels of fertilization far exceed N removal in the harvest, suggesting that substantial loss of nutrients to the environment is possible. The following discussion will suggest some simple practices that can help reduce fertilizer usage in broccoli and cauliflower production.

Crops draw N from several sources: a) fertilizer; b) residual soil nitrate-nitrogen (NO₃-N); c) in-season mineralization of soil organic N; and d) NO₃-N in irrigation water. Accounting for NO₃-N in irrigation water is simple. To calculate how much N you are delivering in the water you apply, multiply the PPM NO₃-N in the water by 0.23 to calculate pounds of N per acre inch of water; well water with a NO₃-N concentration of 10 PPM contains 2.3 lbs. N/acre inch. Assuming an average irrigation efficiency of 70 percent, that means that 70 percent of this NO₃-N could be counted as fertilizer equivalent. As a practical matter, the N contribution of water with NO₃-N concentration less than 10 PPM can be ignored. However, wells that are at least 15 PPM make significant N contributions on a seasonal basis, and this contribution should be included in your fertilizer management.

In-season mineralization of soil organic matter is also reasonably predictable. Tests we have run on dozens of coastal vegetable soils show that from the time of first side-dressing until harvest approximately 1-2 percent of organic N in the top foot of soil is likely to be mineralized (converted into NO₄-N or NO₃-N by soil microbes, available for plant uptake). For a sandy, low organic matter soil that may only amount to 30 lbs. N/acre; for a higher organic matter soil (like that common in the Blanco district) it may amount to twice that much.

The rate of soil N mineralization can be much higher in the first weeks following the incorporation of vegetable crop residue, but it is typically a month or more before the next crop reaches the stage at which sidedressing is contemplated. Mineralization taking place between crops, as well as residual soil NO₃-N carried over from the previous crop can easily be measured by soil testing before sidedressing.

Soil testing allows you to estimate the amount of available N already in the root zone; this can help tailor your sidedress decisions to fit field-specific conditions. In sampling more than 100 coastal vegetable fields just prior to the first sidedressing, I have found that soil residual NO₃-N varied from near zero to as high as 80 PPM in the top foot of soil; since an acre foot of soil weighs approximately 4,000,000 lbs., that range was equivalent to 0-320 lbs. available N/acre. Clearly, knowing how much residual NO₃-N is present can help guide sidedress decisions.

Through a series of more than 30 experiments conducted in commercial vegetable fields we have found that any time soil residual NO₃-N is above 20 PPM, sidedressing can be delayed without affecting crop growth rate. Furthermore, when soil NO₃-N was below the 20 PPM threshold, sidedressing only enough to bring the level up to that threshold worked well.

Most growers sidedress broccoli and cauliflower at least twice. By employing pre-sidedress soil nitrate testing (PSNT) before all sidedressings, unnecessary N application can be minimized. PSNT is less useful early in the spring, when winter rains have usually leached most NO₃-N from the root zone; chances of finding substantial residual NO₃-N levels are slim. However, by May or June significant residual soil NO₃-N levels are commonly found. If soil testing at all sidedressings is too labor-intensive for your operation, concentrate on testing prior to the first sidedressing. It is at that time that the biggest sidedress reductions can usually be achieved.

Putting the pieces together, N use efficiency can be significantly improved by following these practices:

1. Develop a general fertilization plan for each crop that realistically reflects normal field requirements. My experience suggests that for early spring or late fall/winter crops, 180-240 lbs. N/acre, delivered in a small preplant application and two sidedressings, should be sufficient for cauliflower or broccoli under most coastal field conditions. About the only reason that more N would be required is if excessive leaching from rainfall or careless irrigation took place. For crops grown during warm weather months a total of 160-200 lbs. N/acre is a reasonable target range.

2. Adjust these general guidelines on a field by field basis, based on the contribution of N from irrigation water, and pre-sidedress soil NO₃-N testing, particularly at the first sidedressing.

Based on what we have seen with our field trials, the average reduction in fertilizer usage could reach 100 lbs. N/acre per crop for some growers. More detailed information on efficient N management for cool season vegetables is available on the Web site of the UC Vegetable Research and Information Center located at http://www.vric.ucdavis.edu.

Source: Vegetables West/May 2001 issue. Reprinted by permission.