Deficit irrigation affects fresh-market tomato yields

A drought-inspired experiment in reducing irrigation of fresh-market tomatoes over the last six weeks before harvest to below crop water recommendations appears to have shown that a good idea taken too far does not work.

Processing tomato growers routinely reduce water to 40 percent or even further below Et levels in the last five or six weeks, but an attempt to use a similar strategy in three Merced County fresh-market tomato fields using both drip and furrow irrigation reduced yields significantly, and also increased the cull rate.

"Deficit irrigation treatments in the last six weeks of the season, between 42 to 84 days after transplanting, significantly reduced yield in all years and locations," said Scott Stoddard, University of California Cooperative Extension farm advisor in Merced County. "The sunburn fruit and cull percentage increased with deficit irrigation."

The reason late-season irrigation cutbacks routinely work well in processing tomatoes but not at all in fresh-market tomatoes can be traced back to the very different stages of the two crops six weeks before harvest.

"It didn't work because of the timing we imposed the deficit irrigation," Stoddard said. "We fully irrigated until six weeks after transplanting, and then imposed light to severe deficit irrigation. I started the deficit irrigation when the plants were blooming, setting fruit, going through rapid growth stages."

While fresh-market tomatoes six weeks out are setting fruit that will be harvested green, processing tomatoes that far from harvest have already sized fruit that is beginning to turn red.

"In processing tomatoes, the deficit irrigation is after you set the fruit," Stoddard said. "In processing tomatoes, you already have green fruit when you cut back on the water. In fresh-market, you harvest green; the fruit is not fully set six weeks out. It was pretty clear it cut the yields."

Merced-area grower Joe Scoto practices late-season deficit irrigation of his processing tomatoes, when he finds the market makes it worthwhile to grow that crop, but had little luck trying that strategy on his fresh-market crop.

"It did not work," said Scoto, who deficit-irrigated plots of tomatoes on his ground outside Merced as part of the Cooperative Extension study. "You can cut back near the end with processing tomatoes and it works. With fresh-market tomatoes, when you cut the water off, the fruit just melts."

In one plot at Scoto Brothers, Stoddard reduced furrow irrigation by 25 percent beginning six weeks after transplanting, and yields dropped by 7 percent.

When he reduced irrigation even more, to just 70 percent of the standard practice, yields dropped to less than 85 percent of the rest of the field, sunburn jumped to better than 10 percent, and even worm damage increased.

"He was really pushing it, and we're already pushing the envelope on the amount of water we apply," Scoto said. "We use the Et levels to irrigate."

Results were similar at two other fields where late-season deficit irrigation was tried as a strategy for coping with water shortages.

Drip irrigation was cut back from seven days a week to just six, or even five, each of the last two years at Live Oak Farms in the Le Grand area.

The first year, the 15 percent late-season irrigation cutback reduced yields by more than 20 percent, and the more drastic 30 percent cutback nearly cut yields in half.

The consequences were a little less drastic in 2017 at Live Oak Farms, but the reduction to six days of drip still cut yields around 15 percent, and the more ambitious reduction to just five days further dropped yields to 29 percent below the grower standard.

Deficit irrigation did save water at the drip tomato plots at Merced College, as much as 10 inches with the most drastic program of just 40 percent of Et over the last six weeks of the season in 2015.

But that water savings came at the expense of unacceptable drops in both yield and crop quality, Stoddard said.

There are other important differences in irrigation of fresh-market and processing tomatoes, in addition to the long fruit hang time that provides a window for water cutbacks of the canning crop.

Processing tomato yields have increased 40 percent statewide this century as growers have almost all installed and learned how to use buried drip irrigation systems.

While many growers have invested in drip irrigation for fresh-market tomatoes, a $200 million crop statewide, they do not use the technology as exclusively as has become the practice with processing tomatoes.

"There is less drip in fresh-market than in processing tomatoes," Stoddard said. "It's maybe two-thirds drip, and one-third furrow in the fresh-market, but some growers use both. They tend to use furrow early in the season because it gives you a faster start. The drip helps you through the hotter months. There are some growers who prefer to use furrow all season on fresh-market."

Early season growth is important in Merced County fresh-market tomatoes, because growers have to plant late enough to avoid frost yet still be able to harvest in time to hit a market window beginning June 20 or even a little earlier.

(Bob Johnson is a reporter in Davis. He may be contacted at bjohn11135@aol.com.)