By Marni Katz Assistant Editor

Irrigation management may be the key to controlling alternaria late blight in pistachio as chemical fungicides alone are showing little promise in deterring the common disease. University of California Water Management Specialist David Goldhamer said manipulating irrigation is becoming more critical to disease control as traditional pesticides grow less effective and less available. A new project sponsored by the California Pistachio Commission is showing that buried drip irrigation not only limits the prevalence of the alternaria fungus but reduces the impact of the disease on nut quality and yields. Converting to drip is best done in the winter when it is less likely to interfere with pruning and cultural practices.

Alternaria late blight is caused by the fungus Alternaria alternata, and while it affects plants of all kinds, pistachios are very susceptible. The blight is considered a "cosmopolitan" disease which means it lives and thrives in the environment around the plant, such as the air and soil, and therefore is difficult to control.

While pistachio growers this year received a welcomed Section 18 emergency registration for Rovral for alternaria late blight, no fully registered products offer 100 percent control. Rovral, applied in two July applications at the full dosage of 2 pounds per acre is effective, but there is no guarantee that control measure will continue to be available.

Alternaria blight is considered the second worst problem to pistachio growers next to verticillium wilt, and is becoming more and more prevalent every year, according to Plant Pathologist Themis Michailides at UC's Kearney Ag Center in Parlier.

Michailides said Kerman pistachios are particularly vulnerable to alternaria, although all varieties are susceptible. The blight shows different symptoms for different varieties, although the initial symptom to commonly occur is a premature defoliation of leaves.

Lesions begin to develop on leaves shortly thereafter. and in mid-August symptoms move to the nuts. Blackened hulls usually indicate the shell and nut meat will be stained or moldy. The critical period for blight development is from Aug. 11 through Sept. 30 when higher humidity and dew at night create colonization of fungus spores. Fruit is also more susceptible to the disease in August when it is mature.

Nichols Farms in Kings County, Calif., decided to look at subsurface drip irrigation to reduce the disease in one particular orchard that showed severe alternaria late blight. Ranch Supervisor Johnny Starling said the farm was losing up to $500 an acre to blight, and therefor decided to make the costly transition.

The 80–acre orchard, Sierra View Farms in Hanford had been on flood irrigation, but heavy soils with poor infiltration left standing water that not only made cultural practices difficult, but created a humid environment ripe for the development of alternaria fungus. Trees also began to show signs of water stress because not enough irrigation was reaching roots.

Nichols had tried the usual remedies, including gypsum and cultural practices, with no avail. Nichols Farm volunteered to convert half the orchard to drip irrigation under a controlled experiment in cooperation with the University of California. Five replications of 12 rows were put into drip while another five replications randomly remained under flood irrigation. Each 10–acre plot had from 500 to 1,000 trees.

Drip was installed in April 1994, and since then Nichols has seen reduced conditions on the drip blocks. Those same block have increased edible in–shell nut yields this past season by 10 percent over rows that remained on flood irrigation.

Starling said that while increased yields are not dramatic, the ranch is confident enough in results to convert the entire 80–acre block to drip this year.

"We‘re pleased enough that we‘re going to go ahead with it, although we didn‘t see the ‘Gee whiz, this is the best thing since canned beer results we thought we might see,"Starling said.

Nichols Farms shanked in thick–wall drip tubing at a depth of about 28 inches and 5 feet away from the tree row. Some pruning had to be done to gain tractor access to orchard middles.
‘If you wet the surface it defeats the primary purpose of having buried drip‘

David Goldhamer, UC Water Management Specialist

Starling said in a report last year that he would have used shanks that were 6 inches longer to compensate for the uplift of the compacted soil and help work through impacted roots. Some roots were 3 inches in diameter and Starling said roots were commonly a half–inch thick.

Roots can play a major obstacle in converting a mature orchard to underground drip. Nichols used Geoflow drip tubing that has Treflan herbicide incorporated into the emitters to help prevent root intrusion, but roots continued to create a challenge. Excessive root pruning from the shanking affected canopy growth during the installation season. With initial impacts to canopy and roots, it appears conversion should be done during an off year in the alternate-bearing cycle.

The pump was a 100–horsepower turbine moving 1,400 gallons per minute. Low–flow emitters with a 0.6 gallon–per–hour flow were used to reduce the chances of surface wetting, but still some water did make it to the surface.

Starling said he would bury the drip tubing farther down in future applications to keep water from seeping to the surface. While the subsurface drip virtually eliminated free-standing water in the orchard, some surface wetting occurred largely as a result of the heavy soil conditions that forced water up where soil had been worked to lay drip tubing.

"If you wet the surface it defeats the primary purpose of having buried drip," Goldhamer said. Nichols hoped gypsum would help water move laterally rather than vertically. Installation of the drip optimally could be done at a rate of about 15 acres per day, given a normal 10-hour day and tractor speed of about 2 mph, Starling said. In reality, it would have been much less work for Nichols to convert the orchard entirely to drip. Starling said the mainline should be installed, back-filled and watered in by flood or rainfall during the winter when it is less likely to interfere with pruning and cultural practices. Despite the challenges of installing the drip system, Goldhamer, said the results have so far been impressive. Both plots were irrigated with 29 inches of supplemental water. Field conditions – particularly humidity, dew formation and air temperature, which are pertinent to alternaria late blight development - were monitored by data loggers installed throughout the orchard for 1994 and 1995. Statistics and field data show that those rows under drip irrigation had lower relative humidity, shorter periods of dew formation and higher air temperatures in the pistachio orchard. As a result, the incidence of alternaria late blight on leaves at harvest was only 20 percent of leaves sampled, compared to 55 percent in flood rows. More importantly, the percent of nuts infected at harvest were 22 percent for drip rows, compared to 51 percent for fruit in blocks irrigated by flooding. Nut staining, a major symptom of late blight, was generally less in drip blocks. Overall, harvested nuts were 200 pounds per acre higher in drip blocks. In 1995, total edible harvest of split in-shell nuts weighed in at 1,813 pounds per acre for drip, compared to 1,614 pounds per acre for flood. During the off-year 1994, when drip was first installed, edible splits were 1,099 pounds per acrein-shell for drip and 952 pounds per acre for flood. As an added, although unexplainable, benefit, subsurface drip resulted in more split nuts and fewer blanks. In 1995, splits were 82 percent for drip and 66 percent for flood irrigation. Goldhamer could not explain this phenomenon nor the fact that nuts shook easier from trees on drip. "A lot of the physiological precursors to yield seem to develop better in drip than in flood," he said. "Why that is, we don't know."

Starling said the solution to late blight control will likely be a combination of chemical controls, water management and cultural practices. such as cover crops and chiseling between trees to improve water penetration.

"I‘m convinced water management is a major component and could be the key," he said.

Besides disease control, other benefits of having orchards on subsurface drip include fewer weeds and access to the orchard to spray or prune without worrying about flooded rows or the hazard of microsprinklers.

Nut Grower, December 1995