Galls & Burls Explained…Sort of

Trees and other woody plants often have large or interesting swellings on their trunks or branches.   The cause is often difficult or impossible to determine.  Possible causes include fungi, bacteria, insects, mechanical or environmental injury, or genetic mutation.  The terms gall, tumor and burl are commonly applied to describe these abnormal swellings. 

Galls and tumors can be any size or shape and may occur on both woody and herbaceous plants and plant parts.  The swelling occurs as cells divide more rapidly than normal (hyperplasia) and/or due to excessive cell enlargement (hypertrophy).  Burls are generally considered to be large woody swellings that are basically hemispherical in shape.  They often bear many buds and sometimes sprouts.   The burls of black walnut, coast redwood, sugar maple and black cherry are highly prized by woodworkers for their beautiful swirling or ‘bird’s eye’ grain.   This relatively small burl from an apple tree (cause unknown) has an interesting surface pattern and interior grain showing bud traces.

Burl from an apple tree trunk.

 

Tiny brown lines are bud traces.

An individual tree may have one or many swellings.  On this maple tree, the many swellings are of unknown origin.  Often, a tree with large or numerous galls will decline earlier than a tree without them. 

The most common bacterial gall disease is crown gall caused by the bacterium Agrobacterium tumefaciens.   This soil-borne bacterium enters the roots of the host plant through wounds caused by planting, cultivation, frost heaving, insects or nematodes.  The bacteria, upon attaching to the plant cell walls, send DNA that causes production of plant growth hormones into the plant cell where it is incorporated into the plant cell chromosome.  Affected cells begin to multiply at an uncontrolled rate, resulting in visible tumors within 2-4 weeks.   More than 600 plants are susceptible to crown gall.  One of the most common, where galls occur on both roots and stems, is Euonymus, shown in the photo. 

Crown gall of Euonymus.

 

Examples of galls caused by fungi include azalea gall (Exobasidium vaccinii), black knot of plum and cherry (Apiosporina morbosa), and Fusiform rust of pine (Cronartium quercuum).    More information on these diseases is available by clicking on the name of the disease. 

 A close up of a leaf gall on azalea . (Plant Disease Diagnostic Clinic, Cornell University)

Black knot of plum and cherry.

 

 Fusiform rust (USDA Forest Service – Region 8 – Southern Archive, USDA Forest Service, Bugwood.org)

Insects and mites cause some very interesting galls on leaves as shown in the photo.  These usually cause little damage to the host plant or tree and control measures are not normally recommended.  A new theory is being explored by scientists that the swellings associated with these arthropods may in fact be caused by bacteria transferred to the plant tissue during feeding.   Fascinating! 

Hickory gall phylloxera.

 

Galls can be caused by cultural, mechanical and environmental factors including graft incompatibility, wounding, and freeze injury.   Galls on some conifers that vary from small to huge (several times wider than the trunk) are thought to originate when the trees are young seedlings from a single cell and enlarge for many years.  Low temperature injury is suspected, but not proven, as the cause. 

J Allen

 

What causes ringspots on Pachysandra?

Pachysandra is a relatively problem free plant and is widely grown as a ground cover in shady areas.  The most common disease is Volutella blight caused by the fungus Volutella pachysandrae.  Dark blotches form on the leaves and the stems.  These increase in size until the plant is killed.  Plants may be killed in patches within a bed.  This disease is most common when plants are stressed by winter injury, insects, transplanting, mowing, drought or exposure to full sun. 

A relatively new disease of Pachysandra in the northeast is caused by a virus known as alfalfa mosaic virus (AMV).  The virus was first reported on Pachysandra in 1982 in New Jersey and is the only virus known to occur on Pachysandra.  Symptoms include chlorotic ring spots and blotches on the leaves.  This virus is transmitted by a number of aphid species including the green peach aphid and others.  It is also transmitted via seed or infected sap. 

Alfalfa mosaic virus has a broad host range that includes over 600 plant species in 70 families.  Symptoms vary quite a bit depending on the host and can include wilting, white spots, stunting, ringspots, mottling, mosaic patterns or necrosis.  Symptom expression is also affected by the weather, strain of the virus and the age of the plant when infected.   Alfalfa plants can be infected without showing any symptoms.  Plants growing near alfalfa fields are at the highest risk of infection. 

Control of this disease is not always necessary.  Prevention by avoiding planting hosts near alfalfa fields is the most effective.  Managing this virus by controlling the aphid vector population has not been successful.  In the case of Pachysandra, removal of symptomatic plants as they appear is the best approach.   Avoid wounding adjacent healthy plants to prevent sap transmission of the virus. 

Scorch can also occur on Pachysandra and is caused by environmental factors including winter injury or drying.  No treatment is necessary; the plants should recover once new growth appears in the spring.

Scale insects may infest Pachysandra.  The three most common are Euonymus scale, San Jose scale and Oystershell scale.  Control measures for these insects include insecticidal soaps, horticultural oils and conventional insecticides.  Another option for severe infestations is to mow the Pachysandra and collect the clippings to physically remove the scales.  New growth will develop from the stolons. 

JA

 

End of season garden chores.

Lately the calls to the UConn Home and Garden Education Center have been about preparing for winter. People want to prune trees while it is still warm. Pruning for most trees is best done while dormant, in winter cold. Trees really don’t need to be pruned at all, just let them grow into their natural shape and size. If you have a tree or shrub that has outgrown the spot where it is planted, than replace it with a more appropriately sized plant. I have seen many houses where the Rhododendron maximum has taken over entire picture windows! I can only wonder how the view from inside looking out appears. Those plants need to be ripped out and replaced with something that has a much smaller height once it reaches maturity.

Lawns are popular topics on the phone and email contacts. Folks, it is too late to fertilize your lawns now. The plants are going into dormancy. This means they are not actively growing. We know they are not growing because we are not having to mow the grass. If the plants are not growing, they are not moving very much water through their roots up to the leaves. Fertilizer is water-soluble, taken in by the plants once dissolved in the water. So if plants are not picking up the water, they are not picking up the fertilizer either. The fertilizer does not just sit there waiting for the plant to take it. The fertilizer will be washed away with rain and snow, moved down and out of the reach of grass-roots in the spring when the plants wake up and start moving water again. Wait to fertilize lawns until you have to mow again in the spring.

Vegetable gardens need to be cleaned up now. Cut back the asparagus and destroy the stalks by burning or bagging and place in the household trash. Asparagus beetles can overwinter on the dead foliage. Tomato and pepper plants need to be pulled and composted deep in the pile. Clean up all other plant material for a fresh start in the spring. Turning the soil over once will expose overwintering pests living in the soil to the colder temperatures and birds to eat. Try planting a cover crop for tilling in during the early spring. Have a soil test done to determine pH and nutrient levels. Lime can be applied in the fall to work all winter if needed. I plant spinach and cold tolerant lettuces in a bed under a hot cap. It is a frame of plumber’s pvc pipe made into a triangle and attached to 2 by 4′s then covered with painter’s plastic sheet stapled to the wood. We attached hinges to one side of the 2 x 4′s and to the side of the raised bed. Once the seeds germinate, their roots will grow as long as the temperatures are not below freezing. The small plants just hang out all December and January, waiting for the about the last week in February when the days get longer, to start growing into strong, large plants. Picking baby leaves begins in the middle of March. I love picking my own fresh salad while standing in a dusting of snow!

Other end of season garden chores include digging the tender bulbs, tubers and corms for storage in non-freezing temperature. Dig gladioli, dahlia tubers, canna and caladiums. They can be stored in damp peat moss, saw dust or sand. I keep mine in a wooden box in the hatchway  of my home. This area stays between 40 and 5o degrees F, perfect for these plants. I am even trying to save my ornamental sweet potato vine tubers. I will let you know how that turns out! As for herbaceous perennials, I cut them back to the ground to prepare for winter. This eliminates any disease from next year’s growth and hopefully removes overwintering insects, too. Cleaning up the foliage of this year exposes hiding places and homes of mice, chipmunks, moles and voles. Anything to rid my garden of these critters, I will try. Set old-fashioned snap mouse traps near any holes in the ground you find. I have caught mice, voles and chipmunks all in the same small area.

Happy End of the Garden Year,

-Carol

 

Tomato Pith Necrosis

This summer we had an interesting tomato disease in the diagnostic lab.  It’s tomato pith necrosis, caused by the soil-borne bacterium Pseudomonas corrugata. 

Necrosis and wilt symptoms on tomato plant caused by tomato pith necrosis.

The earliest symptom is chlorosis or yellowing of the younger leaves.  As the disease progresses, leaves may wilt and become necrotic (dead).  Infected stems may or may not have visible dark lesions.  The sample received in the lab this summer didn’t have this symptom.  The primary symptom in this case was wilting and necrosis of the upper part of the plant. 

To investigate further and to check for wilt diseases of tomato, the lower stem was cut in half longitudinally.  The characteristic symptom of tomato pith necrosis, a chambered or hollow pith or center of the stem, was observed.  In some cases, this is white as shown in the photo.

White chambered pith in tomato stem.

More advanced bacterial colonization results in the browning and softening of this tissue.  The vascular system may also be brown.  This tissue is in the outer part of the stem and the sample’s vascular browning is pictured below.  Two wilt diseases of tomato caused by fungi, Fusarium wilt and Verticillium wilt, both cause browning in the outer, vascular tissue of the stem, but not in the pith. 

Stem section with both vascular and pith browning.

White and brown (decayed) chambered pith.

One symptom that is quite distinctive but that was not readily apparent in this case is the development of many adventitious roots on the outside of the stem near the chambered pith areas.  Adventitious roots are roots that develop from above-ground plant parts.  Sometimes, the infected tomato plant is able to grow out of this disease. 

 Conditions that favor tomato pith necrosis include low night temperatures, high nitrogen fertility and high humidity.  It often occurs when the fruits are nearing mature green, or just before they begin to redden. 

To prevent this disease, do not over-fertilize with nitrogen and space, prune and stake tomato plants to promote good airflow around them, reducing humidity. 

JA