Evaluation of Soil Steam Disinfestation on Early Growth of Almond (Prunus dulcis) Orchards with High Likelihood of Replant Disease and Confirmed Presence of Parasitic Nematodes

Almond replant disease (ARD) can reduce early and cumulative productive potential of second and later generation orchards. Pre-plant soil fumigation can effectively control replant disease, however increasing regulatory restrictions are complicating use of these treatments. Strip or individual tree site fumigation treatments with methyl bromide or alternative fumigants can provide acceptable control of ARD while significantly reducing the amount of fumigant applied. However, non-fumigant alternatives are needed for areas where grower preference or regulations limit fumigant use. Steam and other forms of heat treatment have been used to effectively disinfest soil in greenhouses and some open-field situations, but have not been tested in almond replant situations. The objective of this research was to develop and test a steam injection system combining heat disinfestation with the principles of spot fumigation for management of ARD. Steam injection augers (24 and 36 inches in diameter) were built in 2010 and trials were initiated in orchards being replanted to almonds throughout the San Joaquin Valley in California. Small plot trials (2–4 trees per plot) were designed to compare varying levels of disturbance and disturbance plus steam. Treatments included the two augers with and without steam, a 4 ×4 ×2 ft. backhoe pit and an untreated control. Large plot trials (24 trees per plot) were designed to compare tree growth following treatment with the 36-inch steam injection auger or conventional fumigant treatments including methyl bromide, chloropicrin and 1,3-dichloropropene. Almond trees were planted 1-2 months after treatments were applied. In the small plot trial near Delhi, increases in trunk diameter after two years were similar (P = 0.05) among the untreated control (31 mm), auger-only (35–36 mm) or auger plus steam treatments (37–39 mm). The backhoe treatment (41 mm) was the only treatment with significantly greater increases in trunk diameter than the trees in the untreated control plots. In the large plot trials at the same site, increases in trunk diameter after two years were significantly greater in fumigated plots (50–60 mm) than the steam and untreated plots (41 mm) which were the similar (P = 0.05). These early growth data at this site suggests that tree site steam disinfestation with a 36-inch injection auger does not provide acceptable control of the almond replant issues in this sandy soil. Tree growth and nematode re-infestation monitoring will continue and nut yield and economic analyses will be conducted as the orchard reaches the bearing phase of its lifespan.