[Malling] of the apple industry someday for the peach industry.”
Two dwarfing rootstocks jointly developed by the University of California and the U.S. Department of Agriculture were recently patented and commercially released, but growers and researchers are still learning about them. Trees on Controller 5 are about 50 percent the size of trees on the Nemaguard rootstock, the industry standard. A tree on Controller 9 is about 90 percent the size of Nemaguard.
The newly released Controller 5 has much going for it, Johnson said. “It doesn’t sucker, seems to be compatible with everything that we put on it, and should do well if it’s planted in a site without a lot of problems with nematodes.”
UC researchers are finding that close attention to water management is needed for some of the dwarfing stone fruit rootstocks. Growers can end up with small fruit if trees are stressed. Dwarfing rootstocks also appear to need more nutrients than trees grown on Nemaguard.
“But if you’ve got a variety where you don’t struggle with size, and you watch the water, they may work,” Johnson said.
The key to using dwarfing rootstocks will be to apply extra nitrogen, watch the water closely, and work with a variety that sizes well.
For many years, Johnson has worked to develop nitrogen fertilization practices that result in good fruit size and quality with minimal groundwater contamination. Research projects have been funded by the Fertilizer Research and Education Program of the California Department of Food and Agriculture through an assessment collected on fertilizer sales.
Past projects have led to significant grower reductions in nitrogen applications, going from 150 pounds per acre of nitrogen applied annually to 25 to 50 pounds of nitrogen per acre through a fall foliar application, and perhaps another 25 to 50 pounds in the spring.
Trials have shown that applying nitrogen to the soil in the fall is a poor choice because nitrogen is taken up when leaves are growing, he explained. Past research projects have also demonstrated the disadvantages of overfertilizing stone fruit orchards, such as brown rot, dense tree canopies, slow-maturing and softer fruit, and bacterial canker.
Hard to get to zero
Johnson began conducting nutrition deficiency studies in 2001 in a sand-box setting that was established to be able to grow trees void of specific micronutrients. The trial, which has 60 trees growing in sand tanks, has been challenging to achieve true deficiencies of micronutrients, he said, because cations found in the fairly hard well water have interfered.
“It’s easier to measure deficiencies of macronutrients like nitrogen, but it gets harder with the minor nutrients,” he said. “It’s hard to get to zero.”
They now use a water softener and regularly truck 5,000 gallons of deionized water from their greenhouse to the sand-box trees. The water softener removes magnesium and calcium. “We’re now close to getting potassium deficiency, but we don’t see obvious symptoms of calcium and magnesium deficiencies.”
He notes that a nutrient deficiency usually translates into a fruit-quality problem for growers. Once trees reach a deficiency, leaves and fruit are sampled at different locations. In zinc-deficient trees, lower zinc levels were recorded in the upper section of the trees. The leaf and fruit sampling shows if nutrient levels are variable within the trees and is used in developing sampling recommendations for orchardists.
Johnson’s deficit irrigation studies on stone fruit are proactive, based on the premise that California’s water future is cloudy at best and at some point, agriculture will be restricted in its water use.
“We are going to run out of water someday,” he predicted, pointing to dropping levels of groundwater, increased pressure on the water supply by the state’s growing population, and lack of new water storage construction. “My motivation is that we will run out of water, though growers aren’t that worried yet because we haven’t had any real droughts.”
He has learned from several years of field trials that growers can withhold irrigation after harvest in early varieties, cutting their water use in half. But while trees survive, the water reduction resulted in double and deep-sutured fruits the following year. And the fruits can have more split pits, he found.
California soft fruit orchardists annually apply between 3 to 3.5 acre-feet of water, he noted. “The research is good information for the day when we only have two acre-feet of water available.”
Thus far, the best deficit-irrigation strategy for growers with limited water, as gleaned from the research data that compared seven different water treatments, is to stress the trees only in June and July, but not in August when flower parts for the next season are forming.