Study shows the drawbacks of overfertilizing

"An extra ounce for good measure"
might work in the kitchen,
but it doesn't apply to fertilizing nectarine orchards.
Greater suseptibility to disease and pest attacks
combined with potential color loss and nitrate leaching
call for greater application care.

By Geraldine Warner

Applying copious amounts of nitrogen fertilizer to nectarine trees does not improve fruit yields or quality, and can make the trees more susceptible to pests and diseases, a team of researchers in California reports. High nitrogen levels can also reduce red color on the fruit and increase the likelihood of nitrates leaching into the ground water.

The three-year research project, which ended last year, involved plant pathologists, biological control specialists, and postharvest physiologists with the University of California.

A certain amount of nitrogen is needed to stimulate the growth of fruiting shoots each year in order to maintain productivity and good fruit size. Although most stone fruit growers in California apply about 160 pounds of nitrogen per acre each year, some apply as much as 200 or 300 pounds in the belief that this will increase yields.

Dr. Scott Johnson, Extension pomologist with the University of California, Davis, said some growers may not have thought about the more indirect effects of fertilization. "You get a nice, lush tree, and it looks great, and it stays productive, but that also makes it a little more susceptible to various insects and diseases," he said.

As a result of the study, some growers have been cutting back on nitrogen applications and have found some benefit, he added.

The effect of nitrogen fertilization on fruit yield and quality was studied in a two-acre block of Fantasia nectarines at the Kearney Agricultural Center in Parlier, California. The trees were planted in 1975 on a 20 by 20 foot spacing and were trained to an open vase system.

Researchers devised five treatments for the study: an unfertilized control, and the equivalent of 100, 175, 250, and 325 pounds of nitrogen per acre per year. All four nitrogen treatments involved an application of 100 pounds of nitrogen per acre in the form of ammonium nitrate broadcast in early September.

The three higher treatments then received more nitrogen in the form of ammonium nitrate or calcium nitrate in the spring. The orchard was irrigated immediately after the fertilizer applications. The treatments began in 1983 and continued through the study.

Yields

Fruit was picked as it matured over a two-week period, and total yields were calculated for each tree. Average fruit weight was estimated. There were no differences in either yield or fruit weight in all the fertilized trees, but both yields and fruit size were lower in the unfertilized trees.

However, increased amounts of nitrogen did seem to affect the time of fruit maturity. The higher rates delayed commercial maturity by about four to five days, based on the ground color of the fruit. Vegetative growth increased with the amount of nitrogen, and the added vegetative growth in the highly fertilized trees increased shading of the canopy, resulting in a longer harvest period.

Leaf nitrogen levels were correlated with the amount of nitrogen applied. Results suggest that too much fertilizer is being applied if leaf nitrogen levels are above 3.0%. In the study, leaf nitrogen levels between 2.8 and 3.0% indicated sufficient fertilization for optimum crop production.

Nitrates in soil samples from areas of the orchards with the three higher rates were higher than from the areas with unfertilized trees or those with 100 pounds of nitrogen applied. The researchers say the potential for contaminating the ground water with nitrates increases substantially when nitrogen is applied at rates beyond those needed for optimum production.

Fruit quality

To study the impact of nitrogen on fruit quality, researchers picked fruit from three different treatments (unfertilized, 170 pounds of nitrogen, and 325 pounds of nitrogen) at commercial maturity based on ground color. There was no difference in the fruit in terms of flesh firmness, pH, acidity, or soluble solids. Nor did the nitrogen level appear to affect susceptibility to bruising, internal breakdown, or flesh softening after up to six weeks in storage.

However, there was a difference in the amount of red color on the fruit. There was more red color on fruit from the lower nitrogen rates. Also, the rate of water loss from the fruit increased with the amount of nitrogen applied. High nitrogen results in denser tree canopies, which shade the fruit, and shading has been reported as the cause of thinner cuticles in many plants. The scientists believe that the greater shading of the fruit in the high nitrogen treatments may have affected development of the cuticle, which acts as a barrier to water movement.

Brown rot

To find out how nitrogen affected susceptibility to brown rot, the scientists sprayed blossoms on the trees with brown rot spores and counted the infected stamens three days later. Blossoms on unfertilized trees had the lowest levels of infection.

They also inoculated green fruit during the growing season, and mature fruit after commercial harvest. Increased fertilization resulted in more latent infections on green fruit and brown rot on mature fruit. It also resulted in more fruit dropped and decayed by brown rot and in more mummified fruit hanging on the trees.

The increased susceptibility to brown rot of fruit on highly fertilized trees could also be linked to the thinner cuticle of the fruit, the researchers believe.

Insects

In commercial orchards, high levels of oriental fruit moth and peach twig borer damage had been noted in trees that produced excessive new foliage because of high nitrogen fertilization.

While both oriental fruit moth and the peach twig borer feed primarily on fruit, they cannot survive in the orchard without feeding on foliage when suitable fruit is not available.

In the study, scientists first of all determined that peach twig borer larvae much prefer to feed on young shoots than old shoots. This was done by bringing a number of old and young shoots into the laboratory, and placing a peach twig borer larva on each shoot. Larvae on the young shoots were able to infest more than 90% of the shoots, whereas less than 10% of the older shoots were infested. This indicates that new shoots provide good feeding sites for larvae, but as the shoots harden off they are less suitable.

To find out how nitrogen fertilization affected the volume of new shoots on the tree, researchers counted the number of new shoots each month. They also looked at the distribution of new and old shoots and infested shoots in the tree canopy. They found little difference in the number of available or infested shoots on any of the trees early in the season. However, after harvest, the effect of nitrogen on vegetative growth resulted in far more available and infested shoots in the highly fertilized trees. This could lead to greater overwintering populations and potential problems the following year.

Fruit damage by the moths was greater in the high nitrogen treatments. There are several possible reasons for this:

First, fruit in highly fertilized trees takes longer to mature, and so is exposed to pests for a longer time. This longer maturing period is more important with cultivars that mature at about the same time as the oriental fruit moth or peach twig borer adult flight, because the moths are more likely to lay eggs directly on the ripening fruit. Hard, green fruit is less susceptible.

Another possibility is that the moths may prefer to lay eggs on trees and fruit in the higher nitrogen treatments.

A third possibility is that moths are able to survive better in the trees with higher nitrogen, perhaps because of greater vegetative growth.