Can Alternative Rootstocks Overcome Replant Disease in Unfumigated, Second Generation Orchards?

Roger Duncan, UCCE Pomology Advisor, Stanislaus County

Although research is ongoing, we currently do not have a good soil test to determine when pre-plant soil fumigation will be economically beneficial to manage replant disease when replanting orchards into second-generation sites. We can (and should) sample for pathogenic nematodes (rootknot, ring, and root lesion), which will tell us if pre-plant fumigation is important to protect new trees from these microscopic, parasitic worms. However, most of the replant problems we see in second-generation orchards are often caused by Prunus replant disease (PRD), not nematodes. Replant disease is a poorly understood soilborne disease complex that suppresses early growth and cumulative yield in replanted almond and other stone fruit orchards. PRD is associated with poor health of the trees’ fine roots and often results in non-uniformity of unfumigated orchards. Although the exact cause of PRD is still inadequately described, it is generally associated with several species of plant-parasitic fungi and oomycetes (such as species of Pythium and Phytophthora). The severity of the disease varies greatly among orchards, but it is observed most commonly on loam, sandy loam, and sand soil textures in California.

Preplant soil fumigation has generally managed replant problems well in most cases, but soil fumigants are increasingly regulated and expensive. Telone II has become a popular fumigant for nematode control but many growers find themselves unable to use Telone II due to township cap limits. In general, chloropicrin, with or without Telone, has been shown to garner better growth responses in replanted orchards than Telone II alone because it is more effective against fungal and bacterial pathogens. Because chloropicrin is less effective against nematodes, mixtures of Telone II and chloropicrin have become popular (i.e., Telone C35).

Until there is a reliable test to determine whether replant disease will seriously affect a new, second- generation orchard, my suggestion is to fumigate. This is based on the results of many UC trials comparing fumigated and unfumigated soils, combined with personal experience observing uneven growth in most unfumigated second generation orchards. This includes all soil types, from the Whitney & Rocklin sandy loams on the far east side of Stanislaus County to the clay loam soils on the west side.

Unfortunately, it is not difficult to imagine a time when regulations and costs will make the use of soil fumigants impractical in California. In 2014, we initiated a trial to see if rotating away from Nemaguard to an alternative rootstock might help with replant disease. The trial is located north of Modesto in a third-generation orchard growing in an excellent Hanford sandy loam soil irrigated with high quality Modesto Irrigation District water. After harvest in 2014, a peach orchard on Nemaguard rootstock was removed, and the site was prepared to replant into almonds. Parts of future tree rows were strip fumigated with Telone C35 at a rate of 46.5 gallons per acre, while other areas were left unfumigated. In January 2015, Nonpareil almond trees, on five rootstocks, were planted in fumigated or unfumigated soil. The rootstocks we tested were Viking, Hansen 536, Empyrean 1, Rootpac R, and Nemaguard. Nematodes, tree growth, and yield were all monitored for six years.

Through the 6th leaf, fumigation has generally improved yield for trees on Nemaguard, Viking, and Rootpac R by a few hundred pounds per acre, although differences were not always statistically significant. Yield differences between fumigated and unfumigated trees are smaller than expected for this site but should still cover the cost of fumigation if the trend continues. Trees on Hansen and Empyrean 1 rootstocks have performed as well in unfumigated soil, as in soil fumigated with Telone C35.

Rootstock choice had a substantially larger impact on tree performance and yield than fumigation in this trial. For example, trees on Empyrean 1 rootstock, growing in unfumigated soil, have outyielded trees on Nemaguard growing in fumigated soil, by 2084 pounds per acre so far. Hansen trees in unfumigated soil have outyielded Nemaguard trees planted into fumigated soil by 1733 pounds. Trees on Rootpac R are substantially smaller than trees on Nemaguard, whether fumigated or not and have yielded roughly half of those on Nemaguard.

Take home messages from this trial:

·       Rootstock has made a much larger impact on tree performance and profitability than fumigation with Telone C35.

·       The superior performance of Hansen and Empyrean in unfumigated soil concurs with earlier trials by Dr. Greg Browne.

·       Although trees on Hansen have performed very well in this trial, Hansen trees in unfumigated soil are supporting larger numbers of ring nematodes (data not shown) and may be more susceptible to bacterial canker. It is very risky to plant a peach x almond hybrid rootstock such as Hansen in soil that will support ring nematodes without preplant fumigation.

·       Trees on Rootpac R are substantially smaller than Nemaguard in sandy loam soils (but not so much in heavy soil), regardless of fumigation. Rootpac R is probably not best suited for sandy or sandy loam soils.

*2019 yield was substantially impacted by bacterial blast in all rootstocks. Bacterial blast is caused by invasion of buds or flowers by Pseudomonas syringae bacteria following sub-freezing temperatures and then warm rain. Rootstock does not generally affect bacterial blast severity.