THOUSAND CANKERS DISEASE AND WALNUT TWIG BEETLE IN A THREE YEAR OLD ORCHARD, SOLANO COUNTY Carolyn DeBuse, Andrew Johnson, Stacy Hishinuma, Steve Seybold, Rick Bostock, and Tatiana Roubtsova ABSTRACT Some walnut trees were found with a heavy infestation of walnut twig beetle in a UC trial of clonal Paradox walnut rootstock in Solano County in the summer of 11. The walnut twig beetle is the carrier of a fungal pathogen Geosmithia morbida which is responsible for Thousand Canker Disease. The trees in the trial were 3 years old at the time and vigorously growing which made this beetle infestation unusual because it usually colonizes older declining trees. The purpose of this trial was to monitor the infested trees in 1 to see if the walnut twig beetle continued to colonize the same trees or moved to other nearby trees. A survey of adjacent orchards of similar age was done to judge the beetle s impact on other orchards in the Solano area. Monitoring included trapping overwintering beetles and testing Geosmithia viability in the young trees over time. The results showed that the beetles did overwinter in the trees but the beetles did not re-infest the same trees in the 1 and very little evidence was found of new beetle activity in the orchard 1. Heavily infected trees showed no growth decline compared to non-infected trees throughout the growing season. The sampling of Geosmithia cankers showed that the fungus could survive over the winter but samples that were taken late in the summer of 1 from healed canker were not positive for the fungus while some cankers that did not look healed did test positive. Survey results in the surrounding orchard did find evidence of the beetle presence but most trees within these orchards and the rootstock block had lower than 1 beetle strikes per infested tree. The results of this study are not conclusive but show a snapshot of the beetle and the disease over this season. OBJECTIVES Objective 1) Monitor and survey the trees in the rootstock trial for evidence of walnut twig beetle and thousand cankers disease that occurred during the 11 growing season. Objective ) Survey other young walnut orchards in the surrounding area for presence of walnut twig beetle activity. Objective 3) Monitor walnut twig beetle emergence in the spring of 1 to evaluate if the beetle overwintered in the young trees. Objective 4) Test for the survival of the fungal pathogen, Geosmithia morbida, in the cankers over time. BACK GROUND Thousand cankers disease is a newly described disease in California walnuts. It is caused by the walnut twig beetle and its association with the fungal pathogen, Geosmithia morbida. The beetle burrows into the phloem of the tree bringing with it spores of the fungus. The fungus then creates small cankers that coalesce into larger cankers. At this time, it has been found in all areas of California where walnuts are grown. The beetle prefers to attack California black walnut but has been found to attack English walnut. Most of the time it is found in English on older California Walnut Board 359 Walnut Research Reports 1
declining trees or in trees whose health has been compromised by another disease such as crown gall or Phytophthora. Just recently it has been found to have attacked a young orchard in Solano County. It is the same orchard that there is an ongoing clonal paradox rootstock project. The rootstocks were planted in March 9 and budded or grafted to Tulare cultivar in August 9. Severely attacked trees had over 3 beetle strikes while most trees had none or less than 1. This research project quantifies the beetle activity in the rootstock trial and young orchards in the surrounding area of this location. PROCEDURES Objective 1- Surveying and monitoring the clonal rootstock trial in Solano County. Three hundred and forty eight trees within the clonal rootstock trial and 73 trees in the guard rows around the trial were surveyed for the number of old walnut beetle galleries (beetle galleries that have been exposed as the cambium and the bark grew in circumference) and beetle holes that are possibly entrance or exit holes were counted on each tree. The tree were high grafted and allowing for about three feet of the rootstock to be surveyed above ground. Both the rootstock and the scion were surveyed separately on each tree. Samples of cankers were submitted to the pathology lab to confirm thousand canker disease by the presence of Geosmithia. Concurrently to this study, the USDA entomologist, Steve Seybold, monitored the flight of the beetle in this orchard. The results of that monitoring can be found in his 1 research report. Objective - Survey three other young walnut orchards in the surrounding area for presence of walnut twig beetle activity. The surveys took place during August 1. Orchard 1 south of the rootstock block is 4 years old Chandler orchard on California Black rootstock. Orchard is northwest of the rootstock block and shares an eastern edge. It is a 4 year old Chandler orchard on California Black rootstock. Orchard 3, just north of orchard, is a 6 year old Chandler orchard on Paradox rootstock. Figure one shows locations of orchards. All three orchards surveyed were nursery grafted trees and had 1- foot of the rootstock above the soil surface. Two rows at each end and rows in the middle of the orchards were surveyed for evidence of walnut twig beetle galleries or holes. Presence or absence was recorded for each tree. Samples of cankers were submitted to the pathology lab to confirm thousand canker disease by the presence of Geosmithia. Objective 3- To discover if the beetle was overwintering in the young trees that showed a large quantity of beetle strikes, a sleeve trap was devised to enclose feet of the rootstock. Six traps were placed in the orchard in late February and early March 1. In refining trapping technique, three types of traps were used, of each type. Figure shows beetle strikes and sleeve trap on tree. The first type trap was lined with pre-applied adhesive sticky paper that is meant to catch mites at a bottom of a beehive. The second type trap had no adhesive trapping surface but instead used a hole that allowed light into the trap. Beetles following the light would fall into a trap jar. The third trap used hand applied adhesive called Stick em Special. The traps were removed from the trees on May 9, 1. Beetles caught in the traps were counted and then sent to the entomology lab to be identified. California Walnut Board 36 Walnut Research Reports 1
Objective 4- Testing was done to observe the survival of the fungal pathogen, Geosmithia morbida, in the cankers over time. Limited numbers of samples were used. Pathology testing as done on samples collected in the fall of 11, late winter of 1, and the late summer of 1. The late summer samples were collected from trees that were heavily infested in 11. Two samples from collected from 5 trees with one sample containing a completely healed over canker and one sample that the canker was not as healed. A comparison sample was taken at this time from an actively infected tree that had evidence of current season beetle holes and bleeding. RESULTS and DISCUSSION Objective 1- Results of the survey of the clonal rootstock trial is shown in Table 1. 155 out of the 348 trees surveyed had some evidence of walnut twig beetle activity. There was no significant difference in number of beetle strikes between the clonal rootstocks. This shows that the there is no beetle feeding preference for one clonal rootstock over another. The numbers also show that an average of 7% of the trees that have evidence of beetles have less than 1 strikes and only 4 % of the trees attacked on average have more than 1 strikes. There seemed to be no pattern to the beetle attacking the rootstock or attacking the scion. Some trees had a higher number of strikes on the scion and some had higher numbers of strikes on the rootstocks. In the trees with the largest numbers of strikes, there were consistently more strikes on the rootstock than the scion. Trees with high numbers of beetle strikes show no observable difference in growth or health at this time. There was no new evidence of beetle activity such as new holes or fresh bleeding in the summer of 1 on any of the trees in the rootstock trial. For whatever reason, like seasonal weather or beetle population dynamics, there were no new or continued attacks from the beetle this year. One tree in the a row adjacent to the trial showed evidence of current season feeding and beetle activity with fresh bleeding and active cankers. Samples of cankers were submitted to the pathology lab did confirm thousand canker disease in this tree by the presence of Geosmithia. Objective - The survey of the walnut orchards in the same area showed high presence of the walnut twig beetle and the thousand canker disease. The survey was done in a presence or absence fashion which allowed a tree with one hole or exposed gallery to be recorded as positive. The majority of the positive trees had only 1-1 strikes on them. No tree in the survey showed signs of poor health due to the beetle activity. A few trees showed a heavy infestation but it seemed to have taken place the years prior and no fresh bleeding was found at any location. Samples were taken and thousand canker disease was confirmed with the presence of Geosmithia. The percent of trees found with beetles strikes is shown in Figure 1. There is a pronounced edge effect with the outside rows have a much high incident of beetle strikes than the center rows. Objective 3- Beetle numbers trapped are shown in Table. Two of the three types of traps failed to catch beetles; the pre-applied adhesive trap and the exit hole trap. The failure is probably due to the fact that given enough time the beetles were able to escape through the screen at the top and bottom of the sleeve. The third type of trap caught a high number of beetles as they emerged from the trunk. This trap was stickier and the beetles were caught and held to the adhesive. The beetles were indentified and confirmed to be walnut twig beetle by the USDA entomologist, Steve Seybold. Both male and female beetles were collected. The conclusion that California Walnut Board 361 Walnut Research Reports 1
can be reached with these data is that the walnut twig beetle did over winter in the trees and emerged the following spring year. The age or generation of the beetle was not identified so there is still a question if these were the adults from 11 emerging for the second year or new generation of beetles from 1. Objective 4- All the canker samples taken from the trial in the fall of 11 and winter of 1 tested positive for Geosmithia morbida fungus. These results show that Geosmithia can stay viable through the winter in Solano County defining it as a perennial canker. Observations through the summer of 1 showed that the young vigorous trees were walling-off the cankers and healing over the old galleries. To test if the Geosmithia was still viable after healing had occurred samples were taken from ten trees, one canker that looked active and one canker that had healed over. A comparison sample was also taken from a tree that had a fresh active infection that showed bleeding. The results from these late summer samples cultured no Geosmithia from the five samples of healed over cankers but two out of five samples from the samples of not healed-over cankers tested positive. The current season infection was also positive for Geosmithia. These results may not be conclusive because of limited sampling but they do lead to the possibility that the vigorous growth walls-off and heals over the canker stopping the canker from enlarging and may eliminate infection over time. This question should be studied more in depth to understand the trees ability to fight the fungal infection. CONCLUSIONS In conclusion, this trial shows that in the Solano County area it is common for young trees to show evidence of walnut twig beetle activity though the number of strikes per tree is most often less than 1 strikes per tree. The young trees growing vigorously continue to grow and heal around the cankers caused by Geosmithia. The infected trees have not shown the decline that the has been observed in older trees. This study cannot conclude that there is no danger of negative consequences to trees in the long run but for now the heavily infested trees seem to be growing equal to their non-infected counter parts. No new re-infestation happened in the year 1. This may be due to the environment or the population dynamics of the beetle. This orchard should be closely watched in the following years to see if it is re-infested or the trees infected in 11 are negatively affected in the long term life of the orchard. California Walnut Board 36 Walnut Research Reports 1
Table 1. The number of trees of each rootstock at the Solano clonal paradox rootstock trial for each category for number of beetle strikes in the 11 growing season. Rootstock VX11 Vlach RX1 Paradox Burbank Number trees in each category of beetle strikes quantity + 1-5-1 1-5 <1 1 5 6 1 3 5 7 5 1 6 1 Orchard #3 58% 38% 89% Rootstock trial 38% Orchard # 5% 35% 55% Orchard #1 68% 34% 57% Figure 1. Aerial map of the area surrounding the clonal paradox rootstock trial in Solano County. Rootstock block is center in solid red with overall percent incident of walnut twig beetle of 38%. Surveyed walnut orchards are shown in red boxes with the percentage of incident of the walnut twig beetle found on both edges and center of each orchard (marked with white line). California Walnut Board 363 Walnut Research Reports 1
A B Figure. Photo A. shows damage to the trunk of the paradox rootstock (high-grafted) with numerous walnut twig beetle strikes. Photo B. shows trunk enclosed by sleeve trap to catch overwinter emerging beetles. Table. Results of the trunk sleeve traps placed on heavily walnut twig beetle infested paradox rootstocks to catch overwintering beetles. Trap Catch Mechanism Pre-applied sticky paper Pre-applied sticky paper Exit hole to trap jar Exit hole to trap jar Hand applied Stickem Special Hand applied Stickem Special California Walnut Board Date of Trap Placement 16-Feb 5-Feb Date of trap Removal 9-May 9-May 9-May 9-May 9-May 9-May 364 Total Beetle Strikes Over Whole Tree 34 4 33 478 547 39 Number of Walnut Twig Beetles Caught 4 7 14 161 41 Walnut Research Reports 1