Diagnosing Pests and Problems

 
Editor's note: Master Gardeners are keeping busy in their own gardens during Napa County's “Shelter in Place” directive. It's spring, it's getting warmer, it's a great time to work in the garden! Here's how Master Gardeners are spending their time:

by Dianne Weyna

I have a very old rhododendron that is doing amazingly well, aside from a few really bad looking clusters of leaves.  These leaves are bleached looking, light white on the top, black spots on the underside.  This plant is on a northwest corner of the house and gets afternoon sun on some areas of the shrub.  Also growing among many other plants in the area are some yet-to-be- picked weeds.  So far rainfall has been the only water in this area.  Still, the plant is set to flower, and I should probably fertilize soon because the bag of special rhododendron fertilizer says to apply after the flowers have set. 

I checked some UC IPM pest notes and it could be thrips or lace bugs. Both can be hard to distinguish because of their tiny size.  It could be mites, but there is no leaf drop or terminal bud distortion and mites don't leave black excrement.  I did see a thrips earlier in the year, so I think that is the most likely cause of the leaf damage.

Good cultural care includes removal of nearby weeds that could harbor the pests, taking care of the plant according to its needs, clearing out old flower buds and cutting off the damaged plant parts. It's probably time to start watering. 

Possible white fly and aphid damage

I have miniature roses in pots on my deck which get southern sun.  I did a thorough pruning this winter.  They have good drainage and I have watered them recently, but not fertilized yet.

I see numerous problems on one of the roses, all the others look good.  Some leaves have a brownish, dried looking area in the middle.  There are a lot of dark spots and a white substance on top and underside of the leaves.  I see ants and aphids and an occasional cobweb. Also, the buds look damaged with brownish tips.  Looking through a loupe I have seen a couple of round balls with holes in them and a small caterpillar, possibly a dead white fly. I wonder why there is so much damage on this plant? White fly and aphids could be the cause of the leaf damage, but why the bud damage?  I see some small aphids in the buds.  I looked in ANR pest notes and some diagnostic tools where I discovered that the few round hard bumps I thought might be scale look exactly like mummified aphids, with the escape hole of a parasitic wasp!

Very happy with that, I ventured that the rose bud damage might be Hoplia beetle damage which looks correct, but there are also aphids.  The Hoplia beetle likes light-colored flowers and these roses are light pink, offering another clue about the cause;  I never thought to consider the color of the flower in a diagnosis.  I will look for the beetles and larvae.  There is a light-colored plant next to the rose, so perhaps this plant is harboring the beetles. This is the only rose planted with another flower. 

I will continue appropriate cultural care and check the other plant for beetles and white fly. A stream of water could get the aphids off, but knowing parasitic wasps are at work, I will live with the aphids right now.

My apple tree had a few stems with white “fuzz” all over them earlier in the spring. Thankfully, there is only one stem with this symptom now. It is a full-size tree that I am trying to espalier, with only partial success. I am sure this creates a bit of stress on the tree. I also plan on doing some summer pruning.  In general, the tree seems to be healthy and had many blossoms on it this spring.  So far it has received only rain water, has good soil and drainage and full southern exposure.

The stem with fuzz on it is near some yellowing, curled leaves. I cut one off, dissected it and found fat, small red-colored aphids, leaving red when they are squashed.  I initially thought they could be food for some good bugs and I should leave them alone, but looking more closely I found scale under all the white fuzz.

I'm diagnosing the aphids as woolly apple aphids, and the scale as European lecanium scale.  I have discarded the stem that had the scale, will keep monitoring and possibly spray with a horticulture oil next year if there is an infestation.

 

Informational links:

UC IPM-rhododendron http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/rhododendron.html

UC California Garden Web-Roses http://cagardenweb.ucanr.edu/Landscape_Trees/Roses_are_Special/

UC California Backyard Orchard-Apple http://homeorchard.ucanr.edu/Fruits_&_Nuts/Apple/

UC ANR pest notes:

PN7428 lace bugs http://ipm.ucanr.edu/PMG/PESTNOTES/pn7428.html

PN7429 thrips http://ipm.ucanr.edu/PMG/PESTNOTES/pn7429.html

PN7404-aphids http://ipm.ucanr.edu/PMG/PESTNOTES/pn7404.html

PN7401-whiteflies http://ipm.ucanr.edu/PMG/PESTNOTES/pn7401.html

PN7499-hoplia beetles http://ipm.ucanr.edu/PMG/PESTNOTES/pn7499.html

PN7408-scale http://ipm.ucanr.edu/PMG/PESTNOTES/pn7408.html

UCANR publication #3359:

‘Pests of Landscape Trees and Shrubs', 3rd Edition Author: STEVE H. DREISTADT

Copyright Date: 2016   ISBN-13: 978-1-60107-864-3

Pages  177, 195, 207-208, 210, 245, 351-352, 377

Rhododendron: University of California, ANR Publication 3359, third edition, p. 377, 207-8, 210,245. 

University of California, ANR Publication 3359, third edition 3359 p. 177, 195 351-352.

 

During Napa County's shelter in place directive that protects everyone's health and safety, Napa Master Gardeners are available to answer garden questions by email: mastergardeners@countyofnapa.org. or phone at 707-253-4143.  Volunteers will get back to you after they research answers to your questions.

Visit our website: napamg.ucanr.edu to find answers to all of your horticultural questions.

Photo credits: All photos by Dianne Weyna