Excess nitrogen is not a good thing in stone fruit

Nitrogen can delay stone fruit maturity, but there are trade-offs.

By Melissa Hansen

More is not better in the case of nitrogen applications on stone fruit. High levels of nitrogen will delay maturity, though it may cost growers in terms of reduced color, increased susceptibility to insects and postharvest diseases, and long-term potential for decrease in fruit size.

Postharvest specialist for the University of California (UC) Dr. Carlos Crisosto discussed the impact of nitrogen on stone fruits with growers during Soft Fruit Day held in Buena, Washington. Crisosto is based at the UC's Kearney Agricultural Center in Parlier.

Excess nitrogen applied to stone fruits can be used as a tool to delay maturity--but it also has negative effects, results from several trials in California show. Rates as well as timing of application were evaluated.

In a 12-year-old block of Fantasia nectarines, nitrogen was at rates of 100, 175, 250, and 325 pounds per acre. No nitrogen was applied to the control.

"The effect of nitrogen in pushing maturity is tremendous," said Crisosto, adding that maturity in the most heavily fertilized trees was delayed by eight days compared to the control or five days compared to the 100-pound treatment. Maturity was determined by fruit ground color, according to California Tree Fruit Agreement color chips.

Timing

Similar results were observed from a study assessing effects from timing of applications. Total nitrogen rates of 100, 250, and 400 pounds were applied in the spring or fall or split in two applications.

Timing of application didn't seem to make a difference, he said. "It just seems you add seven to fourteen days to maturity every time you add nitrogen."

The extra nitrogen increased the tree's top growth and shade within the canopy, delaying coloring of fruit. As expected, pruning weights were heavier in the treatments with higher rates of nitrogen.

The increased shading of shoots also resulted in more shoot death and required extra effort during thinning and summer pruning.

Yields

Growers who think they will boost yields by increasing amounts of nitrogen are mistaken, said Crisosto.

Research shows no increase in yield from rates above 100 pounds per acre and no difference in fruit size. In fact, fruit size dropped in treatments receiving high rates of nitrogen in the last two years of the 12-year study, suggesting negative long-term results.

Color

Fruit color also is significantly impacted by nitrogen.

"Red color is very sensitive to nitrogen and begins to drop with only 100 pounds of nitrogen applied in comparison to no added nitrogen," said Crisosto. "The amount of color loss from nitrogen depends on the variety, but you will always lose some color."

Fruit color of Fantasia nectarines, known to be a poorly coloring variety, dropped from 92% red color in the control to 80% with 100 pounds of nitrogen, to 71% (175 pounds).

The long-term trial also looked at calcium nitrate applications in the spring as a way to size fruit before harvest. No evidence from the research supports the idea of increasing fruit size as long as trees have been adequately fertilized (100 pounds nitrogen per acre). Research also did not show evidence that yearly application of calcium nitrate was able to increase calcium levels in leaves or fruit.

"You will have an increase in brown rot if you apply excess nitrogen," warned Crisosto. His statement is backed by a lot of work done in postharvest research.

Nitrogen fertilization has been positively correlated with increased levels of brown rot blossom infection, he stated.

Tissue of fruit receiving high amounts of nitrogen are more susceptible to bruising, infection, and disease. Higher rates of water loss were observed on fruit receiving high rates of nitrogen. Also, the fruit's cuticle density was reduced in the higher nitrogen rates, which may explain the difference in water loss between treatments.

Researchers believe excessive shading of nectarine fruit in the high nitrogen treatment trees prevents development of the fruit cuticle.

Insects

Growers can also expect to see more insect damage in trees receiving high nitrogen applications, Crisosto noted. The California trials show significantly greater fruit damage from Oriental fruit moth (OFM) and peach twig borer (PTB) in the high nitrogen treatments.

Crisosto said the higher insect damage is thought to occur in part because the fruit must hang longer to achieve maturity, resulting in greater exposure to pests. Ripe fruit are more likely to interest egg-laying moths than hard, green fruit, known to be a less suitable host for OFM and PTB.

A third possibility is that the increased shoot growth attracts the moths and favors moth survival more than the unfertilized and lower nitrogen treatments.

Growers must also be concerned with the potential for substantially higher leaching of nitrogen below the root zone in the higher nitrogen treatments. High nitrates in groundwater supplies are a problem in many rural communities.

Crisosto reported that leaf nitrogen levels can be used as a tool to guide fertilization practices. Research supports the recommendation of maintaining nitrogen leaf levels between 2.7 and 3.0% to minimize excessive vegetative growth, enhance fruit color, and prevent excessive leaching of nitrates to ground water without losing productivity or fruit size.

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Excess nitrogen is not a good thing in stone fruit

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