Root rot in rhododendrons: Symptoms and management

DEAR JESSICA: I have a rhododendron that has been thriving for the last five years. Suddenly this fall, some of the leaves began curling up. My husband pruned off the affected branches, but now more branches are wilting. The Internet says it’s always too much water or too little water or planting too deep, but this tree has been growing and blooming like crazy until now. I have noticed several big, mature rhododendrons in the neighborhood that are also dying. Is there a cure?

— Lois Topping,

Glen Cove

DEAR LOIS: It appears your rhododendron is afflicted with phytophthora root rot, caused by an oomycete, which is an organism similar to a fungus. The oomycete thrives in wet conditions and typically strikes when the base of the plant remains soggy for hours at a time.

Symptoms include discolored foliage that wilts, as you’ve pointed out. Infected plants may survive for a few years before succumbing to the disease. If you look closely, you might notice that the bark around the base of the trunk near the soil is dark.

The pathogen can survive for many years in soil if conditions are right, and it can continue to attack plants year after year, including replacement plants. There isn’t much you can do to cure the plant, except remove affected branches as your husband has done, but there are steps you can take to try to avoid a recurrence next year.

Rhododendrons typically do not require supplemental irrigation once established, so don’t water them except for during periods of prolonged drought. Improve soil drainage by core aerating (taking care not to impact roots) and applying 2 inches of mulch around the base of the plant, keeping mulch at least 3 inches away from the trunk.

It’s possible your rhododendron was planted too deeply and it took this long to display symptoms of decline. If soil is mounded over the root crown, gently remove it to expose the area where the roots flare.

Unfortunately, fungicides won’t cure your plant; they’re best used as a preventative, so after taking the above measures, prune affected branches again and hope for the best. Do not fertilize, as nitrogen will just fuel the disease. And if the plant continues to decline, remove it and take further steps to improve drainage and discourage standing water before planting another in its place. A fungicide may protect new plants from the disease.

DEAR JESSICA: A number of years ago, I made the mistake of planting English ivy to camouflage a chain-link fence in my backyard. It spread all over, even wrapping around trees. Last year, I had a professional kill it, and then I had the fence removed because it was impossible to remove the stalks that had intertwined in the chain links. There remained small areas of ivy in my front plantings, which may have been propagated by birds. For some time I have been trying to eradicate these patches with no luck. I used Roundup, diluted per directions and mixed with some Dawn detergent. No luck. I tried Roundup undiluted and Ortho weed and grass killer, and nothing worked. Do you have any advice?

— Anthony Montuori,

Syosset

DEAR ANTHONY: Roundup, on its own, won’t eradicate English ivy. Leaves of the invasive vining plant that often covers trees, houses and fences are shiny and, therefore, impervious to liquids.

I understand your reasoning in adding detergent. Such additions are often referred to as spreader-stickers, or surfactants, and they serve to adhere the liquid to surfaces such as leaves. Unfortunately, English ivy is even tougher than all that.

To ensure herbicides penetrate the foliage of English ivy, you need to create an entry point. This can be done either by roughening up their shiny surfaces using steel wool, similar to the way you might use sandpaper to remove glossy paint in preparation for a new coat of paint, or by stomping on (and breaking) the plant material. Once you’ve created surfaces through which the pesticide can enter the plants’ system, apply it following the directions on the label.

Remember, broad-spectrum products such as these will kill all plants they come into contact with, including plants and trees, so apply with great care and on a windless day.