Almond trees on three peach/almond hybrid rootstocks did poorly against bacterial canker in sandy soils in a fifth-leaf test orchard near Escalon, according to data compiled by University of California researchers.

Roger Duncan, San Joaquin County farm advisor, says 30 percent of the trees on Hansen 536 rootstock took up the canker disorder, while about 10 percent each of trees on two other rootstocks, Bright's and Nickels, also were lost to it.

Symptoms of the disease appeared in the orchard in 2002 and continue in 2003.

Bacterial canker, one of the main economic diseases of stone fruit, has defied efforts at complete control for nearly 90 years. Known as a “top-down” aerial pathogen, it goes first to the foliage before spreading down to the soil line and killing the tree.

Duncan and other UC specialists reported on research projects during the recent field day held at the Darpinian & Sons Ranch, where they profiled eight rootstocks on Nonpareil planted in 1998. The orchard was previously in two generations of peaches on sandy, flood-irrigated soil and was infested with bacterial canker.

The acreage was fumigated with methyl bromide before the almond planting, and each rootstock was used on 50 trees with Carmel and Sonora as pollinators.

In addition to rootstock tolerance to the bacterial canker complex, they monitored growth, bloom, nut quality and yields.

The rootstocks are Nemaguard peach, the standard, rootknot nematode-resistant choice of San Joaquin Valley growers; Lovell peach, less vigorous than Nemaguard and susceptible to several nematodes; Guardian peach, a Clemson University selection said to be resistant in North Carolina to the equivalent of bacterial canker; Hansen 536, a University of California peach/almond hybrid with vigor and drought tolerance but susceptible to certain root diseases; Bright's, a private peach/almond hybrid development; Nickels, a new UC peach/almond hybrid; and Viking and Atlas, two combinations of peach, almond, plum, and apricot parentage from Dave Wilson Nursery.

Duncan said virtually all the trees became well established, except for 42 percent of those on Viking, which were thought to have been damaged by dehydration in cold storage during rain-caused delays before planting. They were replaced in February of 1999, and those trees have done well since.

“Just in the last two years of the trial,” Duncan said, “we have seen some real differences in bacterial canker susceptibility. Peach/almond hybrids, particularly Hansen 536, seem to be extremely susceptible to it.”

Older trial results

Symptoms of the disease appeared in Hansen 536, Bright's, and Nickels but not in the other five, and Duncan said even greater losses occurred with peaches with the three peach/almond hybrid rootstocks in another trial near Ceres.

Despite the poor performance thus far at the Escalon and Ceres sites, he said peach/almond hybrids yielded well in an older trial at Merced planted in 1989. It is on sandy-loam soil, has moderate ring and rootknot nematode pressure, and is sprinkler irrigated.

In 2002, in that orchard's 14th leaf, meat weight per acre on Nonpareil was 3,256 pounds with Bright's and 3,044 pounds with Hansen 536, vs. 2,466 pounds on Lovell.

“We are trying to find a rootstock that has a higher yield efficiency, meaning that it yields more per given size of tree,” Duncan said.

“We will be looking at some 30 rootstocks from around the world. Some may do very well in a bacterial canker location; some of them will fail miserably. Maybe 10 years from now we will have some very different rootstocks to choose from.”

Blow-over facts

In another related study in Kern County, Duncan said the severe rain and winds of early March 2001 blew over 58 percent of the sixth-leaf trees planted to Nemaguard, while only 13 percent of those on Bright's and 9 percent of those on Hansen 536 were uprooted.

“The peach/almond hybrids are larger trees with larger root systems, they are supposed to have better anchorage, and this trial demonstrated that.”

Pathologist Bruce Kirkpatrick from UC, Davis told growers the field day the Pseudomonas syringae pathogen that causes bacterial canker is weak on its own and depends on predisposing stresses.

Among those are ring nematode infestations, especially more than 500 per liter of soil, which feed on root hairs. Inadequate soil nutrition, especially nitrogen deficiency, causes more disease. Trees on sandy soils with poor nutrient and water holding capacity or those on hardpan where trees get wet feet are in greater danger of infection. Acidic soils also promote the disease.

Symptoms include gummosis originating at the margin of cankers on the bark, a profusion of suckers, and water-soaked, reddish-brown streaks with irregular shaped margins on the cambium.

Kirkpatrick said bacterial canker, unlike Phytophthora species, does not invade the roots but stops at the soil line, where it girdles the cambium, leading to scaffold losses and eventual death of the tree.