JARS v54n3 - What Are Those Spots?

What Are Those Spots?
Bill Steele
West Chester, Pennsylvania

July 1998: three or four people asked if I knew what was causing the spots on azalea leaves. I didn't know. I didn't have them in my nursery, but I had no doubt that I could find a quick solution. Over a year later I still don't know.

The spots are irregularly and randomly spaced yellow dots from the size of a pinhead to 1/16th inch (1.5 mm) in diameter. They appear near the end of June to the beginning of July on current new growth. It is interesting to note that later new growth, on the same branches, does not have the spots. They are not limited to azaleas but are also on lepidotes (small leaf rhododendrons) and elepidotes (large leaf rhododendrons). The spots are not new. One person who has them recalls seeing them on a plant in Robert Gartrell's garden about twenty-five years ago. He made a remark about the nice variegated leaves. Bob Gartrell's response was, "Look closer. It's not a variegation."

Apparently whatever is causing the spots does not harm the plants. One person, who doesn't believe in spraying, has had the spots for at least ten years. The spots that appear around the end of June have faded or disappeared by fall. The cycle is repeated again the following year, with no evidence of stress to the plants. Occasionally damage from other sources is confused with the spotting. In a few instances the plants that have spots have been in a state of decline. Upon further examination the cause of the decline was found to be the result of azalea bark scale, spider mites, or something else not associated with the spots.

I am only aware of the spotting occurring in parts of Long Island, northern New Jersey, the mainline area of Philadelphia, and an isolated case on two adjacent plants in a garden in central New Jersey. It has just started to appear on azaleas in two arboretums in the Philadelphia area.

August 1998: a sample was sent to the Plant Disease Clinic at Pennsylvania State University. The clinic staff said it was not a disease. This has been confirmed by other sources. Samples were sent to Greg Hoover, Department of Entomology, Pennsylvania State University; Barbara Bullock, Curator of Azaleas, National Arboretum; and Bill Miller, The Azalea Works. None had seen this type of damage before. Barbara Bullock sent her sample to Scott Aker at the National Arboretum. Bill Miller sent his sample to Dr. John Neal, a retired entomologist from the United States Department of Agriculture Station at Beltsville, Maryland. They had not seen this type of spotting before and could only conjecture as to its cause.

End of May 1999: cottony masses, approximately 1/16 inch (1.5 mm) wide by 3/8 inch (9 mm) long began to appear on the underside of the leaves of azaleas, lepidotes, elepidotes, and some trees. One person remarked that they were on everything except peonies. Some masses were even on the side of a building! I contacted everyone I knew who had leaf spotting in 1998 and they all had the cottony masses. Samples sent to Greg Hoover confirmed they were egg masses, each containing from 500 to 1,000 lemon-yellow eggs. Because there were no adult females present he was unable to identify them any further than some type of soft scale.

Spots on azalea leaves Egg sacs on azalea leaves
Spots on azalea leaves the size of a pinhead to
1/16th inch in diameter.
Photo by Bill Steele
Egg sacs on azalea leaves.
Photo by Bill Steele

End of June 1999: the egg sacs started to hatch and the spotting began to occur on the leaves of the new growth. Again I contacted everyone I knew who had the egg sacs. The spotting was just beginning to appear on their plants also.

As the spots were forming I sent samples to Greg Hoover, Babara Bullock, and Bill Miller. Barbara Bullock sent her sample on to Ethel Dutky and John Davidson, Department of Entomology at the University of Maryland. They said the crawlers and egg sacs were from a Pulvinaria type scale (cottony camellia scale). They could find no evidence of insect damage causing the spotting. Because the sample they received had been sprayed with Cygon (a very potent insecticide, not usually recommended for azaleas) they suspected it was an injury resulting from this spraying.

Bill Miller sent his sample to Dr. John Neal. Dr. Neal theorized that the adult scales had left the host plant and laid eggs on azaleas and other plants. When the crawlers hatched they attempted to feed, causing the spots, but they were unable to feed so they either died or found a suitable host plant. This seemed like the answer until I spoke with the person in central New Jersey. The egg sacs and the spotting were limited to the same two plants and didn't spread. For the past two years when the crawlers started to hatch he sprayed with Malathion and he feels this has helped to control the spreading.

Greg Hoover said that scale does not travel very far. When the crawlers hatch, they must feed in forty-eight hours or they will die. He could classify them no further than a Pulvinaria type scale.

Bud Gehnrich, past president of the American Rhododendron Society, a resident of Long Island, New York, has also been trying to find the cause of the spotting. He contacted Jim Thornton, past president of the Azalea Society of America, who lives in Georgia. He had not seen them before and could only surmise what had caused them.

Bud Gehnrich also sent spotted leaves to Margery Daughtrey and Daniel Gilrein from the Cornell Cooperative Extension on Long Island and Jim Stimmel, Pennsylvania Department of Agriculture. They found some scale crawlers on the leaves, but the positions of spots and crawlers did not coincide in all cases, so the scale could not be ruled the perpetrator.

Dan Giirein and Margery Daughtrey speculated that it might be Exobasidium burtii, a relative of the fungus that causes the pinkster gall on azaleas. However, according to the Compendium of Rhododendron and Azalea Diseases, "as Exobasidium spots age the spots become covered by a white fungus growth and eventually turn brown from the center out." These spots do not turn brown. In the fall they actually turn a pale green as if they were attempting to take on chlorophyll.

Dan Gilrein felt it is conceivable that the spotting may be a reaction to scale feeding, but this is by no means certain. He also said, "Scale insects probably find new host plants a number of ways. We found newly hatched cottony maple scale crawlers blown from a Norway maple onto roses and herbaceous perennials about a hundred feet away. The crawlers can also move onto new plants via "bridges" where plant parts overlap. Animals can play a role: hemlock woolly adelgid crawlers are known to be transported incidentally by birds. Infested plants can also be introduced to landscapes and be the source of problems in new areas. Adding to the situation is the fact that locally (on Long Island) we have had unusually high populations of scale infestations on landscape plants over the last several years all around Long Island, raising awareness and interest (and perhaps increasing observable symptoms of infestation or injury) related to scale problems."

Margery Daughtrey remarked that people have been asking her about the spotting for twenty years, but lately they are becoming more prevalent. Barbara Bullock wondered if it could be a virus and suggested that I contact the Agdia Laboratory in Indiana. I contacted Dr. Henn at the Agdia laboratory. After I had described the spotting to him he said it is probably not a virus because a virus would continue and not allow later new growth to be unaffected. He suggested I contact Dr. Gary Simone at the University of Florida. Dr. Simone said it did not sound like a virus. I sent him a sample. Dr. Simone's response typified the response from most experts. "No pathogens could be observed on or recovered from symptomatic tissue. Observed symptoms must be attributed to either environmental or cultural stress. No evidence of disease. Spots are not consistently associated with insects - nor is there evidence of feeding wounds. I have not seen this malady before and have no explanation, for it is based upon the sample and phone conversation."

Steve Schroeder, a member of the Board of Directors of the Azalea Society of America, and also proprietor of Holly Hills Nursery in Indiana, felt it might be ozone damage. When I sent him a sample he said he had not seen it before and it was not ozone damage.

At this time we have eliminated: jettisoned jet fuel, solvent from roofing compounds, ozone damage, pollutants from a local industry, a reaction to an insecticide or fungicide, a disease, a virus, feeding of adult lace bugs, and white fly damage.

Even though entomologists have not found any insect damage connected with the spotting we feel there is some between the egg sacs that appeared on the undersides of the leaves at the end of May and the crawlers that hatched at the end of June.

I would like to express my heartfelt gratitude to the lady who has allowed me to come into her garden and take branches from her plants to send to almost everyone I know within the eastern United States. Entomologists have difficulty isolating what is causing a problem when the sample has: lace bugs, red spider mites, white flies, mealy bugs, azalea bark scale, predatory thrips, and probably a host of other insects. After each sample was analyzed I would tell this beleaguered lady they haven't found the cause of the spotting yet, but you should spray for... I think she has worn out at least three sprayers. She sprayed in March with Horticultural Dormant Oil, in early May with Decathelon, and in late May hired a professional. He sprayed with Cygon. Apparently there is a form or concentrate that may be used on azaleas, and this was what he used. There was no damage to the plants. She was devastated when the egg sacs appeared at the end of May to early June, three to five egg sacs on the undersides of the leaves of many plants. She didn't give up. After the eggs hatched she got her trusty sprayer out again and sprayed the crawlers and her spotted azaleas with Orthene and a miticide. At the present time her plants look beautiful, no sign of insects, and I think the spots make a nice contrast.

Although the spots have not caused any visible damage to the plants, we should find the cause and control before it spreads further. This is not a good article to write for a nurseryman who specializes in azaleas and lepidotes. It could scare off a lot of customers, but I don't have these spots, yet. My goal is to find the cause and the cure before I get them and before a lot of other people get them.

Plans this year include sending samples beginning the second week of May, on a weekly oasis, to Greg Hoover and all other interested entomologists. This will hopefully result in a positive identification of the scale that is laying the eggs in the cottony masses. I find it difficult to imagine that there is not a trap available for this purpose. The lady who has been kind enough to open her garden for me may find fly paper stuck to the underside of a lot of leaves.

Unanswered questions: Where does the scale that lays the egg masses come from? What does it eat? Why is there no visible damage to any living plant in the area? Where does it go? What causes those spots?

Bill Steele is a member of the Valley Forge Chapter. Bill and his wife, Ellen, have a small nursery in West Chester, Pennsylvania, specializing in azaleas and lepidote rhododendrons.

by Chase Dooley