Potassium is a critical nutrient in vegetable production in the Salinas Valley. In nearly all key vegetable crops that are produced here, the amount of potassium removed in the harvested crop is similar to or exceeds that of nitrogen (Table 1). Potassium dynamics in the soil are distinctly different from nitrogen, and the need for it can be assessed by a soil test. Ammonium acetate extraction is the most common technique for assessing potassium availability in the soil. Some laboratories use the Mehlich-3 test, which gives numerically a similar result to the ammonium acetate method. In general crops growing in soils with test values above 200 ppm do not respond to potassium fertilization (Table 2).  In general, the decomposed granite soils on the eastside tend to have the lowest potassium levels in the Salinas Valley, while clay loams and clays tend to have higher values (Table 3). However, due to fertilization practices there are some exceptions noted in the 2010 survey of Central Coast soils.

However, there are some details about potassium that need to be kept in mind; the standard soil tests for K (ammonium acetate and Mehlich-3 analytical methods) provide  an index of available potassium and not a quantitative measure of soil potassium content. Other cations in the soil solution (calcium, magnesium, sodium, ammonium) can compete for plant uptake, and therefore it can be helpful to look at the percent potassium of exchangeable soil cations. Cation competition with potassium is not a problem if potassium makes up more than 3% of exchangeable cations. In soils where potassium makes up less than 2% of exchangeable cations, potassium uptake may be restricted. Table 3 shows examples of soils with relatively high potassium levels; in some of these soils potassium is relatively low as a percent of exchangeable cations and may be of concern.

Potassium uptake by plants is affected by the density of the root system of the plant. Factors that affect root development affect potassium uptake: rooting depth, irrigation system (drip vs sprinkler), soil compaction and root disease. In general, any production practice or unfavorable soil condition that reduces rooting density will reduce potassium uptake. An advantage of drip irrigation over sprinkler is the ability to deliver a concentrated dose of potassium by fertigation at the time of the season when demand is highest.

Certain soils have a tendency to fix potassium and make it unavailable or only slowly available to plants.  Soils that contain large amounts of the silicate mineral vermiculite have the ability to trap the potassium ions between layers of the crystalline structure of the mineral, thereby making it unavailable to plants. Vermiculite is generally associated with soils derived from granite, but the type of granite affects the levels of vermiculite. In addition, the age of the alluvium and the degree of weathering impacts the quantity of vermiculite in the soil. Very young soils may not have had sufficient time for all the parent material potassium (in mica and hydrous mica) to weather out of it, and will not have a significant K fixation potential. On the other hand, in very old soils, the vermiculite may have weathered to a different type of clay (smectite) which does not have the proper crystalline structure to fix potassium. Soils with high levels of potassium (e.g., > 200 ppm) are likely low in vermiculite and have little or no potential to fix potassium.

Potassium fixation potential has been identified in some soils used for cotton and other crops on the east side of the San Joaquin Valley but has not been examined in other parts of California. In a recent survey of a small number of soil samples from Salinas Valley vegetable field, none were found to fix potassium in a UC Davis laboratory test for this. The lack of fixation capacity in Salinas Valley soils may reflect the impact of crop production and past fertilization practices. Crop production may affect the weathering of vermiculite on the surface layer of soil. Fertilization practices over the years result in higher soil potassium levels which can saturate potassium fixing sites in the soil minerals.

Potassium is also capable of leaching from soils. We conducted an evaluation of leaching during winter storms on a sandy loam soil on the eastside of the Salinas Valley. The amount of anions and cations leached was measured using suction lysimeters placed two feet deep in the soil; in addition, we measured the quantity of moisture moving through the soil during each rain event. The data indicates that there was movement of potassium and other ions deeper in the soil profile (Table 4). It is interesting to note that, the relative loss of potassium was much less than other cations. Potassium is bound to the negatively charged sites on clay and soil organic matter and losses of potassium due to leaching are generally assumed to be low rich in these materials.

In general, there appears to be robust levels of potassium in some Salinas Valley soils. This is particularly true for heavier soils. However, crop removal by many Salinas Valley crops is also quite robust. Fertilization with potassium does not have the environmental consequences that we observe with nitrogen and phosphorus and it seems prudent to utilize potassium fertilization programs that replace potassium that leaves the farm in harvested product and to use soil testing to monitor the situation.

Table 1. Nutrient content of Salinas Valley crops at harvest (lbs/acre).

1 – higher nitrogen uptake occurs on 5-6 seedlines on 80 inch beds;

2 – these values are currently being evaluated by a research project

Table 2. Soil adequacy levels of potassium for Salinas

Valley cool season crops

Table 3. Analyses of 14 Central Coast soils for available potassium

(ammonium acetate extraction top foot of soil) in 2010

Table 4. Estimate of cations and anions leached during winter

storm events (winter 2009-2010) under three cover crop treatments

Tipburn of lettuce is a calcium-related disorder which causes the development of necrotic areas on the inner leaves of romaine and other leaf lettuce and on enclosed leaves of head lettuce (Photo 1). The necrotic areas likely develop due to a localized calcium deficiency that causes tissue collapse of the affected cells. There are two key factors that affect the development of localized calcium deficiency: 1) uptake of adequate calcium from the soil, and 2) calcium transport through the plant.

In a study conducted from 2005-2006 Tim Hartz, Michael Cahn and I showed that soil calcium levels in the lettuce growing areas of the Central Coast were optimal for plant growth. Soil calcium levels as measured in saturate paste extract was found to be the best indication of available soil calcium to the plant. The reason for this is that soil calcium levels from saturated paste extracts can be easily converted to concentration of calcium in the soil water, as well as pounds of calcium per acre in the soil water. In the two-year survey of a number of soils in the Salinas and Central Valleys, calcium concentrations in the soil water were found to average about 680 ppm; this value exceeds the calcium levels that are used in hydroponic vegetable production systems (100 – 200 ppm). Only one sandy soil in the survey had calcium levels below 200 ppm.

In the study, the effect of supplementing soil calcium with calcium thiosulfate, calcium nitrate and calcium chloride through the drip system was evaluated. Calcium from these materials was applied at the rate of 10-15 pounds per acre. No impact on tipburn symptoms was observed in these trials. To understand why this occurred, it is important to compare the amount of calcium that was applied to what was already in the soil. Calcium levels in the soil water can be converted to pounds of available calcium on a per acre basis; the amount of available calcium in the soil water in these evaluations was approximately 200 pounds/A, so the fertigated calcium supplied only a small fraction of what already existed in the soil.

Another factor in calcium nutrition of lettuce is the effectiveness of calcium transport through the plant. Calcium moves in the xylem by transpirational flow and is delivered to all parts of the plant this way. Factors that reduce transpiration also reduce calcium availability to leaf tissue and may lead to tipburn symptoms. Leaves enclosed in head lettuce and inner leaves of romaine are susceptible to tipburn because they are not transpiring as readily as leaves in the open which can transpire freely. In addition, root growth of lettuce decreases approximately two weeks prior to maturity which can further limit the supply to calcium to the transpirational stream. Various aspects of the growth pattern and physiology of lettuce varieties affect their susceptibility to tipburn (Tables 1-3). In a study conducted in 2005 and 2006, Sylvie Jenni and Ryan Hayes, showed that, in general, head lettuce types are less sensitive to tipburn than romaine types. This is primarily due to greater breeding efforts on tipburn resistance dedicated to date for head lettuce types.

An additional factor that influences the development of tipburn on lettuce is the weather. Foggy weather that reduces transpiration in the last 6-10 days before harvest is conducive to the development of tipburn in susceptible varieties.

One practice that may have a place in reducing tipburn is the use of foliar applications of calcium prior to and/or during the critical period of lettuce development. I currently have four trials out evaluating this technique. At the recommended application rates of the materials being tested, weekly applications of 0.15 to 0.20 lbs of calcium are applied. The trials are currently underway, but the question will be, can we get enough calcium into the plant at the right time? In addition, can the spray reach the tissue that may develop the symptoms, given that calcium does not readily move in the phloem of the plant?

Conclusions: Lettuce production soils of the Central Coast generally have high levels of readily available calcium. Most coastal soils have well over 200 ppm of calcium in the soil water which is adequate for optimal plant growth. The greater issue for the development of tipburn in lettuce is the variety. In addition, persistent foggy conditions that reduce transpirational flow of calcium to all parts of the leaves in the last 6-10 days prior to harvest will trigger this disorder in sensitive varieties.

Photo 1. Tipburn symptoms along the edge of an inner leaf or romaine lettuce.

Table 1. Head Lettuce: percent of plants with tipburn tested in Salinas, California and Yuma, Arizona (adapted from Jenni and Hayes, 2010).

Table 2. Romaine: percent of plants with tipburn tested in Salinas, California and Yuma, Arizona (adapted from Jenni and Hayes, 2010).

Table 3. Leaf Lettuce:  percent of plants with tipburn tested in Salinas, California and Yuma, Arizona (adapted from Jenni and Hayes, 2010).

Another new race, the 13th, of the downy mildew pathogen (Peronospora farinosa f. sp. spinaciae) of spinach has been found and documented. First identified in January 2010 from spinach in Holtville, California, this race breaks the resistance of several important cultivars. The isolate was initially designated as UA0510C and was characterized with a standard set of differential varieties. Isolates apparently identical to UA0510C have been found in an increasing number of locations throughout California in 2010 and 2011. After careful evaluation of the significance of this development to the spinach industry, the International Working Group on Peronospora (IWGP) has designated this isolate as race Pfs 13. The IWGP is located in The Netherlands and is administered by Plantum NL.

Race Pfs 13 poses a threat to the spinach industry because it is particularly well-adapted to modern hybrids with resistance to races 1-12. The appearance of a new race is not unexpected because hybrids with resistance to races 1-12 have been widely planted over the past few years. Similar developments have taken place when races Pfs 5 (1996), Pfs 6 (1998), Pfs 7 (1999), Pfs 8 and 10 (2004), Pfs 11 (2009), and Pfs 12 (2009) were identified and named. The occurrence of Pfs 13 will clearly encourage the industry to develop and use new spinach cultivars having resistance to races 1-13. A history of the detection of the various spinach downy mildew races is presented in Table 1.

A collaboration of researchers with the IWGP, University of Arkansas (Correll), and University of California (Koike) is monitoring the development of new races of spinach downy mildew on a global scale by collecting and testing suspected new isolates. In this way it is hoped that research findings and conclusions will be agreed upon and better communicated between the seed industry, spinach growers, and other interested parties. For California and Arizona, the Correll-Koike team will continue to receive and test spinach downy mildew samples for growers, pest control advisors, and seed companies. Industry is encouraged to continue to submit downy mildew outbreak samples to Correll-Koike, as such samples facilitate the discovery of additional new races. The Correll-Koike research is made possible by support from the California Leafy Greens Research Board and by active participation by the agricultural industries in California and Arizona.

For more information on this subject you can contact Steven Koike (stkoike@ucdavis.edu), Jim Correll (jcorrell@uark.edu), Diederik Smilde (d.smilde@naktuinbouw.nl), or IWGP chairperson Jan de Visser (JandeVisser@popvriendseeds.nl).

Table 1. Races of spinach downy mildew and year of detection

*One time detection only

Downy mildew of spinach is the most important disease on this crop and results in quality and yield losses.

Speck on coastal tomato. In summer (May through July) 2011, bacterial speck disease of tomato is commonly found in a number of fields in coastal tomato-growing areas in Santa Clara and Monterey counties. No doubt the periodic and prolonged rains earlier this year are a major factor in this unusual development. Symptoms consist of dark brown to almost black spots on leaves and sometimes stems. Leaf spots can be circular or angular in shape and individual spots are generally smaller than ¼ inch in diameter. Spots are visible from both top and bottom sides of the infected leaf and are sometimes surrounded by a yellow halo. Leaf spots can merge together and result in the death of large areas of the leaf. On occasion the leaf spots may be located along the edges of the leaves and result in elongated lesions. Stems and petioles also can develop dark brown to black spots that are irregular in shape but tend to be slightly elongated along the axis of the stem. If the pathogen is splashed onto the fruit, disease symptoms on green fruit will consist of small (generally less than 1/8 inch in diameter), slightly raised, superficial black lesions or specks. Such specks usually are not surrounded by haloes.

Confusion of symptoms. Symptoms of bacterial speck on tomato foliage can closely resemble symptoms from the other bacterial leaf spot disease of tomato, bacterial spot (caused by Xanthomonas campestris pv. vesicatoria). In addition, Tomato spotted wilt virus may infect tomato and also cause circular to irregularly shaped, dark brown spots on leaves, again resembling bacterial speck. For all three of these problems, precise confirmation of the pathogen will require laboratory testing.

The speck pathogen. Bacterial speck is caused by the pathogenic bacterium Pseudomonas syringae pv. tomato. Strains of this pathogen are thought to be host specific to tomato, and two distinct races (races 0 and 1) have been documented. This pathogen is seedborne. Bacterial speck on the coastal tomato crop is usually quite minor in severity; once rains stop and if overhead sprinklers are not used, the disease is rarely seen in the summer.

Disease development. Primary inoculum can come from infested seed, diseased plant debris in the soil, or infected volunteer tomato plants. Infested seed is a particularly important inoculum source; if infested seed is used to produce transplants, these plants are often grown under greenhouse conditions that encourage disease development and spread. The practice of using overhead sprinkler irrigation (in the greenhouse or in the field) can significantly spread the pathogen. Once diseased transplants are in the field, the pathogen can be spread plant-to-plant via splashing water (rain, sprinkler irrigation), contaminated tools and implements, and worker’s hands. Disease development is favored by high humidity (greater than 80% relativity humidity), free moisture, and relatively cool temperatures. Again, the rains that occurred through June 2011 certainly encouraged the development of this problem. Interestingly, for all the numerous tomato samples tested by the UCCE diagnostic lab in Salinas, all samples were positive for speck only; bacterial spot disease was not detected at all thus far in 2011.

Control. Obtain and plant high quality seed that does not have detectable, economically important levels of P. syringae pv. tomato. Use a hot water seed treatment or treat seed with hydrochloric acid, calcium hypochlorite, or other recommended materials. Seed treatments must be applied carefully because hot water treatments can reduce seed viability and germination. Avoid using overhead sprinkler irrigation in the field. Applying preventative spray applications to transplants still in trays or to field planted crops may be appropriate. Before using any pesticides, check with your local Agricultural Commissioner's Office and consult product labels for current status of product registration, restrictions, and use information.

Photo 1. Oval to rectangular leaf spots caused by bacterial speck disease of tomato.

Photo 2. Angular spots on leaf margin caused by bacterial speck disease of tomato.

Photo 3. Angular spots on leaf margin caused by bacterial speck disease of tomato.

Photo 4. Elongated stem lesions on transplants caused by bacterial speck disease of tomato.

Photo 5. Fruit speck symptom caused by bacterial speck disease of tomato.