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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.
9-1-05
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