Rosemary Problem

[faraday]

Any thoughts on what this brown stuff is - are they Scale Insects?

 

 

 

[drtreelove]

No I don't think it's scale.  It appears to be a fungal canker with discoloration, tissue distortion and sap exudation. 

Or the curious thing is the enlarged structure in the center that looks like some kind of strange alien caterpillar . Can you isolate and pick off that structure, or is it part of the plant.

 

For a plant problem diagnosis question like this, it always helps to show the entire plant and growing conditions if you can.  Is it one plant or many? young plants? Newly planted? Are they perhaps over watered? or over-head watered? 

 

"The three fundamentals for successfully growing rosemary are sun, good drainage, and good air circulation."

Rosemary Plants: Care and Growing Guide (thespruce.com)

Edited February 18, 2021 by drtreelove

[faraday]

It's a young plant, well ventilated & in direct sun, & is watered over - head.

 

 

 

[drtreelove]

8 minutes ago, faraday said:

It's a young plant, well ventilated & in direct sun, & is watered over - head.

 

 

 

In that photo the brown bumps look something like a soft bodied scale insect.

[drtreelove]

I haven't seen much rosemary in Thailand, but it is a common plant in the landscapes I have worked with in California. I don't usually see pests or diseases. I don't recall ever having to treat with insecticide or fungicide. But that does't mean it can't happen.

 

Can those brown bumps be lifted off or are they part of the plant tissue? If lifted off, then it may be a soft-bodied scale and you may be able to start some control by wiping them off with a rag. 

 

In any case, don't think chemical control yet, concentrate on the growing conditions.  Cease over-head watering if possible, flood irrigate during dry season infrequently, like once or twice a week when the plant is established. They are drought tolerant and don't like over watering. 

 

Don't over fertilize, which can create excessive vegetative growth and conditons favorable to a pest or disease. 

[faraday]

I scraped some of these nodules off, they don't appear to be bugs. Otherwise it seems healthy.

Thanks for all your help.

 

How's your hip replacement these days?

 

 

 

 

Edited February 19, 2021 by faraday
Spellin'

[drtreelove]

My new hips are great, thanks. I'm back in action. 

 

Since you asked about scale insects, here's some information, and for others who may be interested.  It took me a long time to learn to recognize all the variations, and the difference between soft bodied and armored scale, dead scale bodies and active, feeding adults, nymphs vs adults, etc . And how to distinguish between scale and plant galls or other plant tissue abnormalities, and other insect pests.

 

Soft scales put out honeydew as they feed, some excrete an extreme amount that drips into puddles on the ground. Ants are usually in the picture, farming the scale and feeding on the honeydew.  

 

Armored scales are more stealth, sometimes they are barely noticible until the plant is in decline with severe dieback. 

 

Here's a key statement on management that applies to a lot of plant pest and disease issues:

 

"Scales may be a problem in urban environments due to the absence of natural enemies, and plants may be under physiological stress."

 

That's why I say think first about the growing conditions, soil and water management, avoid physiological stress, like water deficit and drought stress.

 

You can't just spray an insecticide to control scale insects. They are covered and protected, contact sprays won't work. Organic program compatible methods can be used. Besides good management, horticultural oil can be used during crawler stage (immature stage in scale life cycle where nymphs are crawling around looking for their spot to settle in and feed). 

 

What is the difference between scales and galls?

The best way to distinguish scales from plant galls is to use a fingernail and flip scales over. Galls, when removed with a fingernail, usually break off with bark attached. Some scales are host specific, feeding on only certain plant species, whereas other scales feed on a wide range of plant species.

Reference: hyg.ipm.illinois.edu/pastpest/200407e.html

 

Soft and Hard Scales: How Do They Differ? (illinois.edu)

 

Armored Scale Identification and Management on Ornamental Plants | NC State Extension Publications (ncsu.edu)

 

Sternorrhyncha Flashcards | Chegg.com

[cooked]

I have had this problem and have given up on rosemary in Thailand. The plants would usually be OK for a year or to and then over a period of a few weeks start to look like this and die. I tried different sprays, anti-fungal and insecticides, didn't work. I also tried messing with soil composition. I did have the impression that it was a fungal problem.

I came to the conclusion that my part of Thailand doesn't like this plant, and despite its decorative value (both in the garden and on food) I don't really need it. 

I also tried with seeds, which I normally had success with back home, they hardly germinated at all, which may also indicate why this is not a common plant in Thailand, there are so many other herbs that are worth growing anyway.