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Growers have made the shift to dwarfing root stock Figures 1a.
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A Guide to Dealing with Tree Decline in Apple & Cherry Blocks*
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A check treatment from the same block showing severe root die-back symptoms after one month. Figure 3b.
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New white feeders showing early signs of die-back. Figure 4.
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Spacing and management have changed dramatically. Figures 1b.
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Lush white root development after the application of Brassica meal. Note the lack of die-back after one month. Figure 3a.
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Trees can reach the top wire and be in production in year 3. Figure 2.
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Growers have made the shift to dwarfing root stock Figures 1a.
Tree roots are important to the overall health of trees and, although proper management of irrigation and soil nutrition are extremely necessary for healthy root systems, it is also important to manage root diseases. This article will concentrate on soil fungi and nematodes, which form a complex that seriously limits production in commercial orchards, and provide guidelines for dealing with these problems based on three years field work with compost and brassica meal.
In the past soil-related pathological problems were generally associated with the fungus (Phytophthora cactorum) which caused crown rot. Looking at tree decline since the mid-1980s, I see a much more significant problem related to root pathogens, including nematodes. This realisation has become more obvious as growers have made the shift to dwarfing root stock of the M9 size (Figures 1a & 1b).
Most of the research on root diseases is associated with specific apple replant disease (SARD). There are chemical controls for most of the pathogens causing SARD including nematodes, but these are either not available to Okanagan growers or growers are discouraged from using them.
One way to overcome the initial set back of tree growth caused by soil pathogens is to fumigate the soil prior to planting to reduce root pathogens, including nematodes. Traditionally, fumigation has rarely been properly or widely applied in the valley, but it has been shown to be sufficiently effective to overcome the initial dramatic impact of replant disease when needed. When a fumigant is used there are ample management techniques available to sustain the rapid growth in year one and two and allow for significant production in year three (Figure 2).
Getting to an economical level of production in years three and four requires the utilization of chemical tools in most orchards, but this presents a dilemma. Many things that have a negative impact on our environment are not politically popular. Good examples are planes, trains and automobiles – and agricultural chemicals. The specific dilemma to agriculture is how to balance the need to compete in the market with the desire to become more “organic.” We can’t do anything about the planes, trains and automobiles, but we are working on the agricultural chemicals.
The political pressure to eliminate soil fumigation has resulted in a number of research programs attempting to find alternatives to fumigation. This, along with the need to dispose of agricultural and urban waste, has stimulated some local interest in testing compost products and other soil amendments, but none have been sufficiently successful to stimulate widespread use or in demonstrating results comparable to chemical treatments.
Current studies on compost and other amendments* and the excellent soil pathology done by Dr. Mark Mazzola, a USDA researcher in Washington state on brassica meal, emphasize the need for a more sophisticated approach to field diagnostics. Mazzola’s work demonstrates the necessity to rely on chemical backup systems when the organic approach falters.
The list of potential pathogens causing problems in the Okanagan is long and includes fungi such as Cylindrocarpon, Rhizoctonia, Pythium, Fusarium, etc, as well as some nematode species. The techniques to determine specific causes in any particular case are available, but somewhat cumbersome, expensive, and time-consuming to apply. Fumigants are very powerful chemicals yet they vary in their efficacy from soil to soil. Looking for specific bio-controls and much weaker biocides for each of the possible pathogens is somewhat impractical. The emphasis in the current compost study has been an attempt to define the specific curative properties of various soil amendments by coordinating compost properties with pathological characteristics of specific soils. Field trials over the past three years show compost significantly affects the occurrence of die-back in roots caused by the soil pathogens2 (Figure 4). Brassica meal shows a similar impact on root die-back.
Dr. Mazzola’s studies also show positive root pathogen control using brassica meal. This work investigated specific pathogen interaction in the soil to explain the mechanism by which the meal works. This work, and the local experience with compost, encouraged our attempt to seek more practical field diagnostic techniques to assist growers with replant or tree decline problems. Studies in the Okanagan, looking specifically at the effect of compost and brassica meal on root morphology and health, show a consistent and distinct impact on feeder roots (Figures 3a, 3b and 4 - see previous page). Specifically this study demonstrated the ability of the soil amendments to reduce or eliminate feeder root die-back. The precise mechanism for this reduction in die-back is still being investigated.
The BC Tree Fruit Co. laboratory in Winfield is currently looking at compost and soil pathogen profiles in an attempt to coordinate specific soil amendments to specific soils.
Utilizing products such as compost and brassica meal as soil amendments in many cases avoids using synthetic chemicals on the soil and assists in the safe utilization of waste by-products generated by agricultural processing and cattle feed lots. It is clear from these studies the solution to replant disease and tree decline lies in a better understanding of the specific problems in the orchard. Methods to diagnose the problems well in advance of tree failure can only be put into commercial practice if effective techniques are available to assess the soils and soil amendments available.
Work at PARC to develop soil pathogen diagnosis will assist extension personnel in the field.
Growers are advised to talk to their field person or the BC Ministry Extension staff for advice on how to best utilize products available. There are a few simple rules to follow when an orchard tree-decline situation occurs (Table 1). In general I would advise conventional growers to fumigate prior to replanting unless a replant test has been done** and indicates fumigation is not necessary. This test involves the evaluation of a number of treatments and can include an assessment of the soil amendment you choose. Where there is a tree-decline situation, in older apple or cherry blocks, a treatment with a good soluble phosphorus product is advised followed by a surface application of compost. It is clear from the studies in reference 2 and from this year’s work that different soils respond differently to the different compost products available. Whenever any product is used as a soil amendment, intended to treat root pathogen problems, it should be accompanied by root observations. Subsequent treatments may be necessary based on the results of the observations. ■
* The local program studying the impact of compost and brassica-meal on tree fruit root systems was supported financially by the BC Agricultural Environmental Initiatives program, the Southern Interior Development Initiative Trust, the BC Cherry Association, Big Horn Compost of Okanagan Falls, CAS Consulting of Penticton and Turffix Organic Fertilizer of Loydminster, Alberta.
** A replant test prior to replanting should be done the fall before planting. The test is currently available through the BC Tree Fruit Lab in Winfield, B.C.
1. J. T. Slykhuis, APS Compendium of Apple and Pear Diseases, 1991.
2. RDOS Reports: Utilization of Compost for Treatment of Crop Roots, 2011 Evaluation of Compost on Crop Production, Tree Growth, Fruit Quality and Soil Fertility, 2012