Category: trees

Plant Detective: Identifying Black Knot Disease

On By durhammastergardenersIn ornamental trees, plant disease, plant fungi, plant pathogen, trees

By Melinda Heigel, NC State Extension Master GardenerSM volunteer of Durham County

This is the first in an ongoing series called Plant Detective, where we’ll uncover science-based information on strange and mysterious sightings in the plant world.

I often learn the most about the wondrous world of plant life through observation. Taking our dogs for daily walks in the neighborhood, around local parks, and on greenways provides great opportunities to study trees, flowers, plants, weeds, and sometimes what are strange unidentified objects. I jokingly call these “case files,” and they often turn into fun research projects that enrich my understanding our environment. I ran into one such puzzler recently.

(Left to right, top to bottom) Glorious late-winter color of the Japanese flowering apricot (Prunus mume); unsightly black wart-like structure on a blooming branch; closeup of a gall encircling a branch; and proliferation of this material on the blooming tree. (Image credit: Form, in bloom jr CC BY-NC 2.0; Melinda Heigel)

Who doesn’t love the sight of blooming trees in mid-February like this Japanese flowering apricot (Prunus mume)? As my eyes were drawn to the stunning pink flowers against the bright February blue sky, I noticed something a lot less attractive: black wart-like clumps of material on the limbs of the tree. What was this unsightly vision? Turns out it is a pathogen that affects numerous species of the Prunus genus. While I won’t disclose the initial terms I typed into the search engine (hint: it’s often referred to as cat poop on a stick), I quickly discovered this growth was the fungal disease known as black knot (Apiosporina morbosa).

The fungal pathogen Apiosporina morbosa occurs on wild, native, and cultivated plums, prunes and cherries and, according to Cornell University, it’s been identified on 24 species of trees. As in our “investigation,” it can sometimes also target others in the Prunus genus including chokecherries and apricots. While initially discovered in the US Northeast in the early 19th century and thought to be a native pathogen, black knot can now be found throughout all of North America.

Evidence of Black Knot Disease

(Left to right) At an earlier stage in the infection, springtime swellings of fungal tissue often have a velvet-like appearance and are olive green in color. By fall, these structures rupture, harden and turn dark brown to black. As the disease progresses, knots continue to spread girdling twigs, limbs, and sometimes the trunk. If left unchecked, black knot weakens the tree, can make it more susceptible to insects and other pathogens, and can lead to its death. (Image credit: Melinda Heigel)

Black knot is a slowly developing disease, and the first visual evidence is subtle. Generally after a tree has been infected for a year, small light-brown swellings or nodes will appear on woody new growth, often at the base of leaves. The spring and early summer’s warm, moist weather provides perfect conditions for this pathogen to flourish, and the seasonal flush of new leaves makes detection tough. Fungal spores that cause the infection are carried by wind and water.

This fungus overwinters on the tree, and these swellings continue to grow and take on a distinctive spongey, velvety texture and green hue in the spring. As the growing season progresses, the knots elongate over time, some encircling branches and twigs. The galls eventually rupture, harden and turn dark and brittle in the fall. By the second year, more and more galls become evident, making detection easy work, especially in winter once leaves have fallen. In the spring, the fugal life cycle begins again.

In addition to the galls, changes in leaves and bark can also signal black knot disease. While some cultivars resistant to the pathogen can sport healthy green leaves in the presence of galls, leaves on more susceptible Prunus species may appear wilted, brown, or die back as the infection progresses. Changes in trunk appearance like black, bumpy and swollen bark and oozing from cracks in the bark are also potential indicators.

Fall-time view of Prunus sp. with with established black knot infection. The black warty galls are made up of both fungal and plant tissues. (Image credit: Melinda Heigel)

Management of Black Knot Disease

How do homeowners, home fruit growers, or ornamental tree lovers attempt to prevent or mitigate black knot infections in their beloved Prunus?

Vigilant Inspection: Continually monitor tree health throughout the year and look for signs and symptoms year round. While it’s a challenge to initially identify, early detection can provide the opportunity to prolong and potentially save the tree if infected. Look for any swelling, discoloration, or galls especially around newly emerging leaves (where first signs appear). Inspection also comes into play when selecting and purchasing a tree at the nursery. Like with all plants purchases, you want to buy a healthy specimen that has no visible signs of this disease or any other disease.

Location Awareness: When locating a Prunus in your landscape, make sure you know if there are other wild or cultivated cherry or plum trees close by which might harbor the fungus either now or later. Since the fungal spores are dispersed by wind and water, plant any new nursery specimen upwind at the very least. Avoid planting new trees around any site where a known problem exists with black knot among wild or cultivated trees. As an example, a local pocket park in my neighborhood has trees with black knot problems and a homeowner adjacent to the park has just identified this disease on her Japanese flowering apricot trees.

Pruning and Proper Disposal: One of the best strategies for controlling black knot is to remove infected plant material, which helps control future spore dispersal and disease progression. Most experts recommend thoroughly pruning all shoots and branches with knots in the fall and winter. This timing is important because the diseased plant parts are most evident and it’s before springtime spore dispersal, which happens around the time of bud break. Some recommend a second inspection and pruning just prior to bud break.

Make cuts on infected limbs 4 – 8 inches past a gall to ensure removal of affected material. If a large branch has multiple galls or is engulfed with knots, consider removing the entire branch. Sanitation is paramount, so disinfect your pruners or loppers between every pruning cut you make.1

Proper handling and disposal of affected plant material are critical. Take care not to drop any diseased or pruned material, as spores could be transmitted to the nearby ground where they can survive. Likewise, don’t compost or place the pruned material at the curb for yard waste pickup. It’s best to destroy the diseased clippings or bag them and put them in the trash. You should likely consider removing seriously infected trees.

Chemical Controls: While most experts agree that certain fungicides can be an important tool in controlling this problem, chemical controls alone cannot stem disease progression without the pruning and sanitation measures mentioned above. If homeowners decide to explore the use of appropriate fungicides in an integrated pest management approach, note that it can be complex: timing based on weather conditions, temperatures, and bloom stage is critical. Copper sprays applied during dormancy and Neem oil have been shown to help inhibit fungal spore production but not eradicate the disease. If you decide to include the use of any chemical controls, make sure you have properly identified the pathogen, read and follow all label directions for timing, application, warnings, and use of personal protective equipment. Some experts recommend that homeowners with ornamental and fruit-bearing trees affected by black knot leave the spraying to commercial growers and stick to to cultural and sanitation controls only. (See resources below)

Disease-resistant Varieties and Alternatives: When choosing Prunus cultivars to plant in your home landscape or for fruit production, look for disease resistant (not disease proof) options. Talk with your local nursery about what types do best in your area, especially if you live in a hot and humid environment like central North Carolina. If you are looking specifically for spectacular spring-blooming trees for ornamental purposes, a native tree like the redbud (Cercis canadensis) or the serviceberry (Amelanchier genus) can certainly fit the bill.

Keep an eye out next time you are walking. Who knows what intriguing things you might see! As spring and summer are on the horizon, what plant mystery will we solve next?

___________________________________________________

Notes

1–Rubbing alcohol (70% isopropyl alcohol) or a 10 percent solution of household bleach are great options for sanitizing garden tools. See Iowa State University’s Extension and Outreach site for more details. https://hortnews.extension.iastate.edu/faq/how-do-i-sanitize-my-pruners-prevent-spread-disease

Resources and Additional Information

To learn more about growing fruit from the Prunus genus like plums in the home landscape, including great varieties suited to central North Carolina, consult the online version of the North Carolina Extension Gardener Handbook.

https://content.ces.ncsu.edu/extension-gardener-handbook/15-tree-fruit-and-nuts#section_heading_9399

For more detailed information on black knot (Apiosporina morbosa), check out the following online factsheets from West Virginia Extension and Cornell University.

https://extension.wvu.edu/lawn-gardening-pests/plant-disease/tree-fruit-disease/an-introduction-to-black-knot

http://plantclinic.cornell.edu/factsheets/blackknot.pdf

Article Short Link https://wp.me/p2nIr1-437

Assistant plant detectives Baby and Cole are always on the case. (Image credit: Melinda Heigel)

The Budding Orchardist: Training and Scouting

On By durhammastergardenersIn edibles, Gardening information, trees

By Jeff Kanters, Master GardenerSM Volunteer

“The only true wisdom is in knowing you know nothing” –Socrates

Spring arrived in central North Carolina in an unusually wet and cool fashion. The young orchard trees, after several months’ dormancy and given ample spring rains, began waking up quickly in February. The pear trees started blooming first in early March, followed in April by the plum, peach, apple, and Asian persimmon trees and the service berry shrubs. Then, surprise! An unexpected March freeze scalded most pear flowers resulting in lost or misshapen developing pear fruit.

Training the shape of the young trees began in earnest: As buds broke and new growth was observed sprouting from trunks and branches of the trees, we pinched off growth that was oriented upward or inside the limbs to reduce future growth of unwanted developing branches, which will keep the plums and peaches open in the center and keep the tree canopy open for the apples and pears. As always, our goal was to increase air circulation to reduce fungal diseases as well as enhance sunlight penetration into the tree for optimal future fruit production.

As new limbs rapidly grew in April and May, we initiated the placement of spreaders on new limbs and clothes pins on the tips of the green growth of pears and apples specifically to orient new branches outward from the main trunk at a 45-to-60-degree angle to form a stronger crotch angle that better supports future fruit, minimizes limb breakage, and opens the canopy. We plan on leaving the spreaders and clothespins in place until the respective branches have hardened by early fall. Another tool for training trees is the use of weights to pull down a growing branch. There are many types of weights one can construct for this purpose. One of our Durham Master Gardeners assisting in the orchard, Eric Wiebe, created small limb weights that he gifted to the orchard. He has blogged a terrific DIY for making these weights for training tree limbs, which will be posted here in July.

Spreaders, clothespins, and weights applied to pear tree. Photos courtesy of Jeff Kanters

First came spring, then came flowering, and then came fruit!  The orchard trees bore their first fruit since being planted in 2021. Given the youth of the orchard trees – about 3 years old – and the smaller stature of the apple and pear trees, we chose to thin any developing fruit to about two to three fruit per tree to observe the development and keep energy directed at tree growth and vigor. We had fruit development on one pear, three apples, and two peach trees that we then thinned of developing fruit.

This spring into early summer we faced numerous disease and insect pest challenges: In April we discovered Fire Blight infection on two apples [Malus sp.], ‘Arkansas Black’ and ‘Black Twig.’ Fire blight is a bacterial disease [Erwinia amylovora] that attacks susceptible apples and pears and some related genera. It favors the cool wet spring conditions that we had in central North Carolina this spring. Fire blight starts by affecting the newest growth on the tips of branches, causing a terminal wilting, and a drooping and eventual blackening of the tip’s leaves and stem that takes the shape of what is termed ‘shepherds hook.’ It moves down the stem and branch quickly.

Fire Blight on apple. Photo: Jeff Kanters

When blight is observed at this stage, the best and only means of control is to prune off the affected tip down 8-10 inches from tip to clean tissue using a sterilized [70% alcohol or Lysol] hand pruning shears as soon as possible. We pruned out developing fire blight as quickly as we could, bagging the limbs and removing them offsite from the orchard. Our fear was that if the disease progresses, it can spread to the trunk and throughout the tree and eventually kill it – and these were young trees. Some higher production orchards may use an Erythromycin dormant spray on susceptible trees at early spring bud swelling to prevent or minimize the development of this blight. However, this is an expensive option and if used too frequently the bacteria may develop resistance to the antibacterial. Although there are notable resistant varieties marketed to consider, no variety is immune from this disease in the Carolinas if the conditions are right!

Sadly, for those wondering, the efforts to curtail the blight on the small ‘Arkansas Black’ apple were not successful.  With over half of the tree quickly succumbing to the disease and as the trunk appearing affected, we lifted and bagged the tree, and removed it offsite from the orchard. The other ‘Black Twig’ apple appears to have overcome the disease after pruning out the blight and is putting out healthy new growth. More good news, none of the other orchard pears or other apples displayed fire blight this season.

Rusts make their debut in the orchard: In April, to our dismay we discovered rust infections of developing fruit on one pear [Pyrus, sp.] tree and on all the developing berries of the two serviceberry [Amelanchier, sp.] shrubs. The rust on the serviceberry and pear fruit is fungal, and like other trees in the Rosaceae family, such as apples, requires an alternate host such as Eastern Red Cedar [Juniperus virginiana, sp.] and other junipers, and/or Flowering Quince [Chaenomeles, speciosa] to complete its life cycle.  The infected berries and fruit from the service berry shrubs and pear tree were all removed, bagged, and discarded away from the orchard. The leaves of both plants appeared untouched. The orchard area where the rust seemed prevalent is next to a forested area that may harbor numerous Eastern Red Cedar trees as well as block air flow next to the orchard trees infected with the rust. This creates an environment that may be fostering fungal infection. There is a greater plan to open up the forest understory away from the orchard to increase air flow and to remove any eastern red cedar in the vicinity. Dormant spraying of the fruit trees is planned in late winter to help reduce fungal infections in general.

Rust on service berry fruit and pear fruit, courtesy of Jeff Kanters

San Jose Scale makes an appearance in the orchard: As some of the apples started producing fruit, in early May we observed that the two apples developing on the ‘Black Twig’ apple displayed a grouping of small, rounded, purplish haloed spots, primarily at the bottom of each young apple. After further research we believe the apples were infected with San Jose Scale. The scale, typical of many scales, has a crawling stage and sedentary stage. Please refer to the link on this specific scale, its life cycle, and how to best manage it below. We promptly removed the two young apples, bagged them, and discarded them away from the orchard. We are monitoring the leaves, stems and twigs for signs of potential scales, but have not noticed any yet. One way that this tiny scale might be transmitted from one tree to another is by wind during its crawling stage, as the females never crawl far before attaching and creating a scale like cover. 

San Jose Scale on apple. Photo: Jeff Kanters

Yet again, it is Japanese beetle [Popillia japonica] time: Like last year, we experienced our first infestation of Japanese beetles of a biblical proportion! We hand picked off over 3,000 beetles over the course of six weeks. To our horror, they had quickly skeletonized about 60% of the leaf canopy of several plums during that infestation. Fortunately, all the trees put out new growth and recovered. Whew!

Japanese Beetles. Photo: Jeff Kanters

This season, we were prepared, at least psychologically, for their arrival. They began arriving the week of May 22nd, about the same time as last year. We set up a schedule each week to have someone at the orchard in the morning as many days as possible to manually pop off the beetles into a cup of soapy water. We had found that early morning removal was the most effective, as the beetles were still sluggish and could be swept into the soapy water more easily. We are thrilled to report that the numbers through June were significantly less than last season. Five weeks into their arrival, we have removed roughly 850 beetles overall, a 70% drop in numbers compared to last year.

Persimmon Psyllids infest the Asian persimmon trees: This spring as the persimmon trees began their growth cycle, the growing tips of most branches of all persimmon trees displayed a curling, distorted and stunting of the new leaves not seen before. My first reaction was to panic and fear the trees were all suffering from a systemic disease of some sort. Affected leaves from all the trees were collected and submitted to the North Carolina State University State Agricultural Laboratory for inspection and analysis. To our amazement, the culprit identified was the Persimmon psyllid [Baeoalitriozus diospyri]. Try pronouncing that scientific name!

Apparently, these tiny insects function much like aphids, sucking plant juices. Their populations in cool rainy springs, again like we had here, can explode. The good news is that they do not like warm, hot weather and as summer kicks in their populations decline and the trees typically recover, no spraying needed. Lesson to the wise, I now always carry a pocket magnifying glass in the orchard. Refer to the below link on this pest from the NC State Extension.

Psyllids on persimmon. Photo: Jeff Kanters

As the weather has warmed up heading into summer, we have begun monitoring moisture levels on all orchard tree berms. When needed, we manually water all trees once or twice weekly. We give each tree a good five minutes of watering out to the drip line. Watering all the trees takes approximately 2-3 hours for one person. An irrigation system is being planned for installation in 2024.

On the agenda for this summer is completing soil testing of all the orchard berms. While most fruit trees prefer a soil pH of 6.0 to 6.5, we have identified a smaller bermed area for potential blueberry shrubs. In that case, blueberries prefer a more acidic soil of 4.5 to 5.0 so a higher soil pH test may require amendments to lower the pH. Anyone’s guess what that amendment may be? Stay tuned.

Also, summer pruning is ahead for applicable trees. Our goal is to complete this pruning process no later than mid-July. We will conduct this mostly to remove upright, inward, and crossing new branches of the current season’s growth back to a side shoot branch. This is termed a thinning cut and does not invigorate the tree near the cut, which is what we want.

Much more ahead on:

  • How summer pruning the trees is done
  • Our ongoing efforts at recognition and management of disease and pest control
  • The application of milky spore to kill those dreaded Japanese beetle grubs that newly hatched this early summer in the soil
  • New dwarf fruit and berry trees and shrubs being considered for a growing orchard that do well in central North Carolina

Additional Resources

Fruit Trees General:

Texas A&M Extension Field Guide to San Jose Scale:

Persimmon psyllids Description and Biology:

Service Berry Rust Description and Biology:

Fruit Tree Pruning Basics: