Advice for the Home Gardener from the Help Desk of the
UC Master Gardener Program of Contra Costa County
Client's Request: Hi. We have a beautiful, delicious, amazing plum tree in our backyard. Every year, we've had so many plums that we've been able to share them with friends and neighbors.
This year, our tree produced very few plums. Many were malformed. I also noticed some fungus-looking growth on the tree. Can you help me identify the fungus and/or suggest how I might fix my tree? Thank you! (pics below).
Help Desk Response: Thank you for contacting the UC Master Gardener Program Help Desk about your plum tree. Your email indicates that your plum tree produced fewer plums than usual this year and that some of them are “deformed”. You also have noticed some “fungus looking” growth on the trunk of the tree.
I have what I believe will be welcome news to you (and to your neighbors who no doubt love sharing the fruit from the tree). Based on the information provided in your email and what we can see in the photos you provided, we do believe that the tree has not developed any serious long term problems.
Let me start with the “fungus looking” growth on the tree. What you are seeing are lichens. Lichens are living organisms composed of both a fungus and an algae. The fungus and algae live in a symbiotic relationship. The fungus receives some food from the algae and the algae some food and protection from the fungus. They are not parasitic on the host tree (i.e., they do not receive nourishment from the host) and are generally harmless to the tree. Lichens grow in a wide variety of shapes and colors, ranging from the gray green and yellowish colors shown in your photos to bright red for some types. Lichens are more noticeable on older trees such as your plum than they are on younger trees because the rapidly growing trunks of young trees tends to disburse the lichens. As indicated in this University of California website, it is possible to remove lichens using herbicidal soaps developed specifically to control mosses, lichens, and algae. http://ipm.ucanr.edu/PMG/GARDEN/ENVIRON/lichens.html Since the lichens are not harmful to the tree, using such soaps is unnecessary and we would not recommend it for your tree.
Now let's turn to the question of why your plum tree produced fewer fruits this year than you have had in the past. There are a couple of possible explanations. One possible factor is the weather conditions that we experienced in the spring months when the plum tree was in bloom. This year, we had a warm and generally dry February and early March. Fruit trees tended to bloom early. Then we started having some regular rain storms and cooler weather. The rain storms may have washed off some of the blooms on your plum tree. Cool, rainy weather also tends to keep honey bees and other pollinators away. Those conditions may have held down the fruit set.
Another possibility is “biennial blooming” which often occurs with fruit trees. With biennial blooming, the tree produces a heavy crop one year, a very light crop the following year, followed by a heavy year, then a light year, etc. Hormones produced in fruit seeds are to blame for biennial bearing. The hormones present in the tree with a heavy crop suppress flower-bud formation the following year. In a year with few fruits, hormone levels stay low, so there are more blooms and more fruit the following year. To interrupt this phenomenon, you can reduce the number of fruits that are allowed to develop in a heavy year by thinning the crop.
Possibly for your fruit tree, there might have been a combination effect—the rain removed some of the blooms and held down pollinators and perhaps the tree was already in a lighter fruit production cycle since you had a good crop last summer.
Finally, let me address the “malformation” of some of the fruit. We don't see much evidence of malformed fruit in the photos you sent. We do see one fruit that looks like it has cracked. Such cracking often is a result of erratic watering while the fruit is developing. If the tree received significant water when the ground was dry, it could have taken up the water rapidly and caused the fruit to split.
Possibly the “malformed” fruit has already fallen from the tree so you cannot provide photos. In that case, we would suspect that the pollination was inadequate. Similar to the small crop this year, the rainy conditions and lack of pollinators during bloom could have been factors for the malformed fruit.
We hope that this information is helpful. If you have further questions, you are welcome to contact us again. Enjoy the plums that you do have; hopefully you'll have a better crop next year. tkl
Help Desk of the UC Master Gardener Program of Contra Costa County (TKL)
Note: The UC Master Gardeners Program of Contra Costa's Help Desk is available year-round to answer your gardening questions. Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA 94523, although we will be moving late July. We will notify you when that occurs. We can also be reached via telephone: (925) 646-6586, email: email@example.com, or on the web at http://ccmg.ucanr.edu/Ask_Us/ MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Biog (http://ucanr.edu/blogs/CCMGBlog/)
Advice from the Help Desk of the
UC Master Gardener Program of Contra Costa County
Response from the Help Desk: Thank you for contacting the UC Master Gardener Program Help Desk, and for resending the photos of your plum tree.
It looks like your tree is heavily infested with Kuno scale (Eulecanium kunoense), which is a common pest in plum trees in the Bay Area. Kuno scale females are almost spherical, resembling beads on stems. They are dark brown and shiny most of the year, turning yellow/orangish during egg production in spring. The nymphs (young scale) are yellow or brown and flattened. They feed on leaves during summer, then go to the branches to overwinter. There is one generation per year. This link will take you information from the University of California on Kuno scale: http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/INVERT/kuno.html. And for more information on scale management in general: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7408.html.
Scales secrete honeydew, a sticky sweet substance that attracts ants. Ants in turn protect the scales from their natural enemies that could help control their population. So, the first thing to do is to control the ants. If the tree's branches touch other plants or structures, prune those branches back so the ants don't have that access route. Use a sticky substance on the trunk to prevent ants' access that way. Wrap the trunk with a collar of heavy paper, duct tape, or fabric tree wrap and coat it with a sticky material such as Tanglefoot. Check the coating every week or so, and stir it to expose new sticky material. If left too long, ants, other insects, and debris will collect on it and obscure the sticky material, allowing access once again. You should also be aware and check to assure that birds occasionally get stuck in this substance trying to eat the trapped ants, etc.
You can also use bait stations to reduce the ant population. Borate-based products seem to be the most effective, as the bait is sugar based and should attract the honeydew-eating ants. Place the station near the base of the tree. Ant baits take a while to work, so it may be a while before you see any results. This link will take you to information on ant management: http://ipm.ucanr.edu/PMG/GARDEN/CONTROLS/antmanagement.html. For more information about ants: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7411.html.
You mus also make sure you are giving good cultural care to your tree, especially with adequate irrigation, to make it more resistant to scale damage. To immediately reduce the scale population, you can prune off the most heavily infested twigs or branches, if that's appropriate for the tree's form. Your pictures look like it is a mature plum that might benefit from a little summer pruning anyway. By opening up the tree, it may increase scale mortality from exposure to heat and predators.
Chemical controls can be used for especially bad infestations. Horticultural oils (narrow-range, superior or supreme), insecticidal soaps, neem oil, or other botanical oils have low toxicity to people and pets, and appropriately applied should have little impact on pollinators and natural enemies. Make sure you follow all instructions on the products' labels, looking especially for which plants not to use the product on. For your plum, spraying with an oil during the dormant season may be the easiest. Foliage spray can be effective to get the nymphs in their crawling stage, but it can be difficult to make sure the product covers all leaf surfaces on a large tree.
To recap: Ant control is the first thing to do. Good cultural care is next. To encourage natural enemies, grow a variety of flowering plants to help attract and support them. Adults of predatory bugs, lacewings, lady beetles, and parasitic wasps live longer, lay more eggs, and kill more scales when they have plant nectar or pollen and insect honeydew to feed on.
It will take several months of efforts (such as controlling ants) or until the next season or longer before scale populations are reduced by biological control. If the level of scales is still intolerable, then consider spraying insecticidal oils to reduce scale populations while conserving natural enemies.
Please let us know if you have further questions.
Good luck with your plum tree!
Help Desk of the UC Master Gardener Program of Contra Costa County (SEH)
Note: The UC Master Gardeners Program of Contra Costa's Help Desk is available year-round to answer your gardening questions. Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA 94523. We can also be reached via telephone: (925) 646-6586, email: firstname.lastname@example.org, or on the web at http://ccmg.ucanr.edu/Ask_Us/ MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog (http://ucanr.edu/blogs/CCMGBlog/).