|Volatile Organic Compounds
Replant disorder/soilborne diseases
Replant disorder is the result of a complex of soilborne microbes and plant parasitic nematodes that build up in the soil during the life of the previous almond orchard, and suppresses the growth of trees replanted into the same environment. For this reason, growers typically fumigate the soil before replanting to eliminate or reduce the pathogens and nematodes in the soil, and choose rootstocks that have some degree of resistance to the disease complex present in the soil.
If soil tests determine that oak root fungus (Armillarila) is present, for instance, choose a plum rootstock such as Mariana 2624, which is the most resistant. Nemaguard and generally all peach rootstocks are susceptible to oak root fungus, bacterial canker, and crown gall. If bacterial canker (Pseudomonas) is part of the disease complex, consider Lovell rootstock, which is somewhat resistant, but Mariana 2624 and peach/almond rootstocks are very susceptible.
Rootstocks vary in susceptibility to the different Phytophthora species; none are resistant to all pathogenic species of the fungus. Thus, the success of a rootstock may depend in part upon the species of Phytophthora present in the orchard. In general, plum rootstocks are more resistant than peach or peach-almond hybrids. Of the plum rootstocks, Marianna 2624 is the most tolerant to Phytophthora. For a more complete list of soilborne diseases in almonds and best rootstock selections, see the UC IPM Pest Management Guidelines.
Nematodes are an example of a pest that can live in soil, water and plant tissue. They are almost too small to see without magnification. While some types can damage plants, others do not.
There are three major species of nematodes affecting almonds: root knot, root lesion, and ring. The dagger nematode is also common and is capable of transmitting yellow bud mosaic virus to almond trees.
Rootstocks for almonds differ in response to various plant-parasitic nematodes. Nemaguard peach rootstock, almond-nemaguard hybrids, and Marianna 2624, which are resistant or immune to most common and injurious root knot nematodes, are susceptible to ring nematode and root lesion nematode, common pests in old orchard and vineyard soils. Lovell peach rootstock is susceptible to ring, root knot, and root lesion nematodes. Other peach rootstocks (S-37, rancho resistant) offer resistance to one root knot species but not to others. Almond rootstock is rated susceptible to root knot, root lesion, and ring nematodes. Use of resistant rootstocks may be limited by soil and horticultural considerations.
When planting or replanting an orchard, be sure to sample for nematodes, especially if the land was previously an orchard or a vineyard.
Depending on the history of disease in the previous orchard, as well as the results of the soil testing, the use of a pre-plant fumigant may be called for. Depending on the pest complex present as well as what is allowed by regulations select the appropriate soil fumigant(s). As the regulations for soil fumigants are changing rapidly, contact your County Ag. Commissioner's office Resource Map and check for updates at ABC website with correct laws & regs for soil fumigants. The following time schedule is recommended, however many shorten the replant cycle by fumigating immediately after orchard removal:
Summer/Fall: Remove trees or vines, destroy residues, and deep cultivate to remove residual roots and break up cultivation pans or soil layering.
Winter/Spring: Fallow or plant grains.
Spring/Summer: Level (if necessary), cultivate, and do other operations required for next year's planting. Dry the soil.
Late Summer/Early Fall: Rip the soil. Apply chosen soil fumigant(s) in compliance with the regulatory requirements, and before soil temperatures/moisture reduce the efficacy.
Winter/Spring: Observe waiting period on fumigant container label; plant young trees on resistant rootstock if root knot nematode is present.
UC IPM Pest Management Guidelines
Seasonal Guide to Environmentally Responsible Pest Management Practices in Almonds