EFFECT OF CROWN GALL ON INCIDENCE OF THOUSAND CANKERS DISEASE Mohammad Yaghmour, Hector Facundo, and Elizabeth Fichtner ABSTRACT One hundred and sixty-seven trees in a Kings County orchard were surveyed for the incidence of crown gall and thousand cankers disease. The incidence of crown gall was 40.7% of the trees infected with crown gall, and 25.1% of the trees exhibited thousand cankers disease symptoms with visible signs of WTB infestation. A total of 41 trees (24.6%) were infected with both crown gall and thousand cankers disease. Healthy trees without crown gall had significantly larger tree diameter compared to trees with crown gall. There was a significant negative correlation between the extent of crown gall on diseased trees and tree diameter. Crown gall was also a predisposing factor for thousand cankers disease. The odds of having thousand cankers disease in the presence of crown gall significantly increased, and if a tree was symptomatic of crown gall infection it had 102:1 chance of being infected with thousand cankers disease. However, when a tree diameter increase by one unit, it decreased the odds of thousand cankers disease incidence by a factor of 0.67:1Because trees with crown gall exhibited statistically lower trunk circumference, the association of thousand cankers disease with trees of smaller circumference is likely not a predisposition based on tree size, but rather predisposition caused by crown gall. OBJECTIVES

1. Survey for incidence of crown gall and thousand cankers disease. 2. To look at the effect of walnut crown gall on tree growth (diameter). 3. Examine the relationship between crown gall and thousand cankers disease. 4. Survey the walnut industry in Kern County.

SIGNIFICANT FINDINGS

1. The incidence of crown gall and thousand cankers disease on surveyed trees was 40.7% and 25.1% respectively, with 24.6% of the trees had both diseases.

2. Healthy trees had significantly bigger tree diameter compared to trees with crown gall. 3. Increase in the percentage of the crown gall at the crown significantly correlated with

decrease in stem diameter. 4. Crown gall and reduced tree diameter significantly correlated with incidence of thousand

cankers disease. Thousand cankers disease incidence increased as percentage of crown gall increased and tree diameter decreased.

PROCEDURES A 18th-leaf walnut orchard in Kings County with a history of crown gall was surveyed for incidence of crown gall and thousand cankers disease (Fig. 1). The trees were Chandler variety planted on seedling Paradox rootstock in 1999. In total, thirteen rows were marked for survey in the orchard.

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Each tree was evaluated for presence or absence of crown gall and thousand cankers disease, and extent of crown gall was recorded as a percentage the crown area exhibiting crown gall symptoms. Tree evaluation was done by visual inspection of each tree for 30 seconds to record the approximate percentage of crown gall and presence or absence of thousand cankers symptoms on the main trunk. Also, trunk circumference was measured approximately four feet above soil level to estimate trunk diameter.

The effect of crown gall on trunk diameter was analyzed using correlation analysis and t-test to compare tree diameter of healthy and crown gall infected trees. The relationship between crown gall and thousand cankers disease incidence data was analyzed using non-parametric statistical analysis. Pearson’s Chi-Square test for independence was used to test for independence of crown gall and thousand cankers disease as a null hypothesis. A logit model was also used to test the effect of crown gall incidence and trunk diameter on the incidence thousand cankers disease and how that affect the odds of a tree being infected with thousand cankers disease.

Also, a survey was designed to collect information about the walnut industry in Kern County and some of the challenges facing the growers and their priorities.

RESULTS AND DISCUSSION

A total of 167 trees were surveyed. The incidence of crown gall was 40.7% of the trees infected with Agrobacterium tumefaciens with visible gall formation at the crown, and 25.1% of the trees exhibited thousand cankers disease symptoms with visible signs of WTB infestation. A total of 41 trees (24.6%) was infected with both crown gall and thousand cankers disease (Table. 1).

Healthy trees without crown gall had significantly larger tree diameter (11.86 ± 0.16 inches) compared to trees with crown gall (10.27 ± 0.26 inches) (Fig. 2). There was also a significant negative correlation between the percentage of crown gall on the tree and tree diameter (p < 0.0001, r = -0.5139). As the percentage and extent of crown gall increased, tree diameter decreased (Fig. 3). We found that 26% of the total variation in tree diameter can be explained by the extent of crown gall infecting the walnut trees (R2 = 0.2641). A previous study in northern California has also reported decrease in tree circumference and cross sectional area with more crown gall (Epstein et al., 2008).

Statistical analysis confirmed that crown gall and thousand cankers disease are not two independent events from each other in this orchard, they are linked and there is a connection between crown gall and thousand cankers disease incidence (p < 0.0001). A logit model revealed that incidence of thousand cankers disease is also significantly affected by crown gall and tree diameter (p < 0.01). The odds of having thousand cankers disease in the presence of crown gall significantly increased, and if a tree had crown gall it had 102:1 chance to be infected with thousand cankers disease. However, when a tree diameter increase by one unit, it decreases the odds of thousand cankers disease incidence by a factor of 0.67:1. Because crown gall-infected trees exhibited lower trunk diameter than uninfected trees, the positive correlation between reduced tree diameter and thousand cankers disease incidence may be indicative of predisposition of crown gall to thousand cankers and not a direct relationship to tree size.

This is the first study evaluating the relationship between crown gall and thousand cankers disease. Observations in the orchards at UC Davis experimental plant pathology station and in commercial

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orchards suggested that trees with crown gall may have higher incidence of thousand cankers disease. However, this study establishes that when a tree has crown gall, it is also more likely to have thousand cankers disease. The results also suggest that crown gall has a predisposing effect on walnut trees increasing the likelihood and the odds of having thousand cankers disease. Crown gall as a predisposing factor can have a negative effect on tree growth and tree diameter contributing to increased chances of developing thousand cankers disease compared to healthy trees. Healthy trees have bigger trunk diameter and as trunk diameter increase the odds of the tree developing thousand cankers disease decrease.

More surveys in major walnut growing regions to test this hypothesis will provide more information on the effect of crown gall on the incidence of thousand cankers disease. Understanding major factors contributing to thousand cankers disease development will provide us with scientific information necessary for future disease management.

For the survey of walnut industry in Kern Co., the size of the industry is very small and orchards are being removed. I met and discussed issues with two major growers in Kern County. The first grower removed 140 acres of walnuts that was planted 1971 with Serr grafted on northern California black walnut. The removed block was irrigated with surface flood irrigation system. Crown gall was also present on several trees. The acreage removed represented approximately 17% of the total acreage reported for 2014 in Kern County crop reports. The decline in price was the only factor for taking the decision to remove the orchard with only production between 3500-3800 pounds per acre. The grower expressed the need for research for new varieties and rootstocks that suit Kern Co. The other grower who responded to the survey during a farm visit is a representative of a major nut growing company. The company planted 620 acres in Kern Co. and expressed the possibility of increasing their acreage in the near future. The new orchard was planted with Chandler and Tulare varieties, and irrigated with microsprinklers (Fig. 4). When asked about the most important topics with high research priorities in orchard management, new varieties and rootstocks, salinity, and irrigation management and water quality were at the top of the list. Crown gall, walnut blight and canker disease management were among the most important subjects to address orchard disease and IPM management research priorities. This effort will be an ongoing part of the orchard systems program to better address research and educational needs of walnut growers in Kern Co. REFERENCES Epstein, L., Kaur, S., McKenna, J. R., Grant, J. A., Olson, H., and Wilbur, W. O. 2008. Crown gall can spread between walnut trees in nurseries and reduce future yields. California Agriculture 63(3):111-115.

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Table 1: Contingency table of crown gall and thousand cankers disease incidence on walnut trees.

Crown gall

Thousand cankers disease

Yes No Total Yes 41 27 68 (40.7%) No 1 98 99 (59.3%)

Total 42 (25.1%) 125 (74.9%) 167

Figure 1. Symptoms of crown gall and thousand cankers disease on walnut trees (cv. Chandler).

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Figure 2. Tree diameter of healthy trees and diseased trees with visible crown gall on tree trunk at the crown area above soil surface.

Figure 3. Correlation analysis between percent crown gall (%) and tree diameter (inch).

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No Crown Gall

Crown Gall

Tree Diameter (In)

y = -0.0299x + 11.901, R² = 0.2641(p < 0.0001, r = -0.5139)

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Tree

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Figure 4. New orchard in Kern County planted with Chandler and Tulare cultivars

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