Question and Answer
Q. I have a rhododendron bush that looks like it is getting rust on the leaves. I think perhaps something is eating on it. What should I use to control this? Thank you. -- Joan Wininger
A. It's important to determine if the rust is a problem or a natural occurrence. Many rhododendrons have a natural, rusty-brown, scaly appearance, particularly on the underside of the leaf. Rhododendrons may also be showing leaf scorch at this time of year. Neither requires treatment.
Rust diseases can occur on rhododendrons, but are rarely seen. If the symptoms you see are due to a rust disease, the problem will be randomly distributed. Small pustules appear on the underside of leaves. These pustules burst open to discharge bright yellow, orange or brownish spores that re-infect the rhododendrons or azaleas. Control of this disease is usually not necessary. Many of the new varieties have some resistance to rust.
Hemlocks and spruces are alternate hosts for rust diseases, so try not to plant rhododendrons near these trees if rust is a problem for you. Also, keep plants vigorous with proper water and nutrition, keep the area free of infected leaves, reduce humidity, improve aeration, avoid excessive leaf moisture and use resistant cultivars when possible.
The fact that something appears to be eating it is probably a separate issue. Many questions should be answered before a diagnosis is given, but look for irregular shaped notches on leaves to signal the feeding of adult black vine weevils, one of the most common pests of rhododendrons.
Black vine weevils are black in color, have a fairly long snout and are about three-eighths of an inch in length. Adult weevils feed on the leaves of yews, rhododendrons and many other trees and shrubs. Notches on these leaves are typical symptoms of feeding damage in late spring and early summer. During the summer months, adults will mate, and the females will lay eggs in the soil located under the host plant. The eggs hatch into root feeding larvae that feed on roots from midsummer until fall. A few of the larvae may overwinter in the soil and continue feeding in the spring. However, many larvae will pupate in the fall, with the adults emerging in late fall and overwintering in plant debris, and then becoming active in the spring.
Black vine weevils feed primarily at night, so you may not see them in action. Also, the damage from a previous year remains on these broad-leaved evergreens, so be sure you're seeing new activity before you choose to treat them. The adults cause unsightly leaves, but the larvae are a more serious threat to the plant's survival. Their root feeding can cause the damaged roots to be unable to take up the proper amount of water and nutrients needed for the plant to live.
Once all of the weevils have matured, apply imidacloprid in late July. This can kill adults feeding on leaves and the larvae that hatch from eggs of adults. Read and follow label directions carefully.
Q. Hope you can give me some advice on ridding the pest that is eating up my plants: mums, sunflower leaves, small mimosa trees and an evening-blooming flower related (I believe) to the Jimson weed or nightshade family. The bugs are a half inch or less in size, black with neon greenish-yellow stripes, and orange head and underside. Thank you for your help, -- Mary Coveyou, French Lick, Ind.
A. It could be the 4 lined plant bug. The damage is very distinctive, with circular brown spots on the leaf tissue where it was feeding. Adults attack a variety of bedding plants and perennials, with plants in the daisy and mint families especially susceptible. Damage occurs in late spring and early summer when the nymphs are active. Nymphs are reddish-orange and are quick to run to the underside of leaves when disturbed. Adults are lime green with four black stripes on the back.
As nymphs and adults feed, they inject enzymes into the plant. Feeding damage appears as small (one-sixteenth inch) round sunken spots on the leaves. Large numbers of this insect may cause entire leaves to curl and wither.
Since the nymphs cause most of the damage, control at this stage is suggested. Small numbers can be dislodged from the plant into a container of soapy water, and large numbers can be controlled with an insecticidal soap or a labeled insecticide.
Q. What can you tell me about a tiny bug that closely resembles a boll weevil that has attacked my hollyhocks the last few years? In my garden books, the only reference I can find that sounds similar is the plum curculio, but the list of affected plants only includes fruit trees. Picking them off by hand is very time consuming, and they seem to always come back. Thank you. -- Jana L. Shaffer, Geneva , Ind.
A. Hollyhock weevils can riddle the leaves. The adults are dark gray with reddish-brown legs and around around one-tenth to one-eighth inch long. The female has a longer snout than the male, so she can chew into the developing ovaries of the flower buds to create holes to lay her eggs.
The larvae feed and grow in the developing seedpods and eat the hollyhock seeds, then emerge as adults through pinhole-sized holes they create in the pods. Adults feed on leaves, creating small holes in the foliage and on buds. They primarily feed on hollyhock but have also been reported on Malva and Lavatera . Infested seedpods should be removed and destroyed. Adults can be treated with carbaryl (Sevin) or Cyfluthrin ( Bayer Multi-Insect Killer).
Q. I am hoping that you would be able to answer a concern of blue patches that occurred on my lawn this spring and how to prevent this. I noticed areas of small patches of bluish-purple grass. I have never encountered this before and do not know what steps to take to correct this situation. It is spreading through my lawn. Thank you for your help. -- Karen Portz
A. The patches are likely natural responses for cultivars or biotypes to cooler temperatures and/or reduced day length and are probably not an issue. If they have continued, bring photographs to your the Purdue Extension office in your county. Sorry I can't be more specific without more information.