Products touted to correct calcium deficiencies in vegetable crops need to be viewed in perspective with variety selection and environmental factors, according to Tim Hartz, vegetable crops specialist at the University of California, Davis.
Calcium shortages show as tip burn on lettuce or as blossom-end rot on peppers and tomatoes. They can be blamed on lack of soil calcium (not typically an issue in California), water stress, or anything that limits movement of the element via transpiration to aerial parts of the plant.
Hartz told a recent vegetable production meeting at Five Points that tip burn, a serious disorder of lettuce, is a prime example of failure of the plant's transpirational stream to distribute calcium.
Older, outer leaves, with their active transpiration, get adequate supplies of calcium, but the younger, internal leaves often draw too little and develop symptoms of browning at the margins.
In peppers and tomatoes, when calcium is not distributed to the fruit, blossom-end rot shows. Water-soaked areas appear around the blossom scar, and then the sites become dark brown and sunken. Dead tissue often causes premature ripening of the fruit close to the sunken areas.
Excessive temperatures and high humidity also impede calcium delivery, regardless of what a grower does.
Last July and August in Salinas, Hartz put out trials observing soil calcium and tip burn in commercial lettuce fields. “We found that locations where evapotranspiration was the lowest during the weeks before harvest were where tip burn developed,” he said.
He explained the standard soil test, the ammonium-acetate extraction method, shows calcium but not the form that is taken up by crop roots. He used the alternative, the saturated paste extraction test, which he said “is not perfect but gives a much better correlation with the actual solution in the soil.”
He noted that much research has been done on blossom-end rot on tomatoes and the only successful practice is to manage irrigation to minimize stress while the fruit are small and in the most rapid expansion phase. Once the fruit are half- or three-quarters developed, stress is not so critical to the disorder.
In peppers, irrigation management is also pivotal when fruit are expanding. But Hartz said he has encountered growers who tend at this point to also apply additional UN-32 or ammonium nitrate. The fertilizer actually suppresses the uptake of calcium and aggravates blossom-end rot.
Pepper spot, another calcium deficiency that shows as black or brown spots, has been historically common to coastal counties, although in recent seasons it has appeared frequently in the San Joaquin Valley as well.
“The solution to this problem is variety selection,” he said, citing results of several commercial field studies. Vegetable specialists once thought most of the older open-pollinated pepper varieties were susceptible to the spot and most hybrids were immune. However, recent trials indicate not only that the OPs are still susceptible but some hybrids are also.
Researchers have recently evaluated calcium-bearing products, both soil and foliar applied, that claim to correct the disorder. Of six pepper trials done with the materials, two showed some improvement, but four showed no effect at all, Hartz said.
The problem persisted, he added, because transpiration limited distribution of calcium to needy portions of the plant.
Even with drip irrigation, now used on an estimated one-third of coastal lettuce, experiments with injected calcium have not conquered tip burn. Hartz said of trials at Salinas, “we nudged up calcium levels statistically a little but not very much. It's not the amount of calcium in the soil but how to get it to move into the plant tissue.”
Although several commercial lettuce varieties have some degree of tolerance to tip burn, USDA breeding work continues in the search for new germplasm for improved varieties.
Melon growers pursuing export markets want firmer fruit to better withstand shipment, and increasing calcium has been one attempt at finding an answer. But Hartz said honeydew trials with drip irrigation at Davis indicated that even with multiple applications of calcium-bearing products, no significant response was realized.
While some marginal advantage was achieved by stressing the melons, firmness, or the amount of force required to puncture a fruit, remained mostly regulated by variety.
In a presentation on bacterial diseases of tomato, Mike Davis, plant pathology specialist at UC, Davis, said tomato bacterial speck is a persistent and costly disease that plagued many growers in the wet spring of 2005.
By contrast, incidence of the moisture-driven disease was nil during the dry 2004 spring. Research on it is difficult because of the erratic outbreaks, perhaps in one of every four or five seasons.
It is different from other bacterial diseases in that it survives in soil, on debris from diseased plants, and on seed.
“We will probably always have this disease because it can survive on both leaf and roots of many plants that are not related to tomato,” he said. “So you can't just blame the occurrence of speck on bad seed or contaminated transplants.”
Management of the Pseudomonas syringae pathogen has also been difficult because it has two races. Race 0 has been most common in California and several tomato varieties are resistant to it, but no resistance is available to Race 1.
Race 1 is increasing, and Davis sampled 32 fields in 2005 across the state and found it to be in 31 of them.
It's been long known that Race 0 has partial but widespread resistance to copper in California. Sprays of copper-containing bactericides offer some control but must be applied before the cool and moist conditions that favor the disease. Tank mixes of copper materials and mancozeb improve efficacy.
In tracking tomato bacterial speck last year, Davis quantified its significant effect on yields. He calculated that a 10 percent disease level claimed about 0.66 tons per acre, which, based on a price of $50 per ton for processing tomatoes, came to a loss of $33 per acre. Although plants grew out of the disease as temperatures warmed in the summer, the loss in marketable fruit was not recovered.