Yellowing leaves with black spots

[roadrunner]

Charlie do you have any idea what is going on here?  This started last winter inside the house and has continued all summer and increased on several plants in the greenhouse. The black spots are on the underside of the leaf initially and are like tiny raised bumps. As the black lumpy spots get larger and increase the leaf yellows rapidly.  Eventually the spots  become visible on the upper side as well.  Once a few spots start on a leaf the whole leaf goes very quickly.  I've been removing these leaves as quickly as I've seen these symptoms.  I've been battling spider mites all year and am wondering if this is related, possibly some secondary disease iniated by the spider mite damage?  Many other leaves where the spider mites are evident by small pin prick size light green spots never evolve into this.  As mentioned elsewhere in the forum I've washed the plants, sprayed soap, and also pyola.

thanks,

dave

[helixturnhelix]

Hi Dave,

I am eager to hear what Charlie has to say, as I have seen the same thing on some of my plants.

[Charlie]

First off, make sure that the black areas do not rub off when wet. Use something - finger, Q-tip, wash cloth - wet it and scrub the black area to be sure that it is part of the plant tissue and not something on top of the leaf surface.

If nothing comes off then it is definitely a leaf disease problem. These are such a rare occurrence on hibiscus in our climate, except in winter, that I've never had to worry about them. Dave, do your roses show something similar? They are more inclined to get such leaf fungus diseases and might be the source of the fungus.

Bart in Kentucky has sent me photos of the most awful looking black fungus I've ever seen on hibiscus. The effected parts of the leaves look like they've been coated in tar. Made me wonder if some tobacco diseases can transfer to hibiscus. I referred him to a professional plant disease specialist who used to be a professor of same specialty in Florida. She might shed some light on his particular problem.

Meanwhile you can contact the County Extension agent where you live and find out how to submit samples of effected leaves to their experts for diagnosis. Some are better than others but if the problem has been seen before in the area they could ID it.

I suppose if it was on my plants I would first try spraying it with Bayers 3-in-1 since it has a fungicide intended to help with leaf diseases. Good air circulation and making sure the leaves are dry by nightfall can also help with fungus problems.

Let us know what happens.

Charlie

[helixturnhelix]

Thanks for the info Charlie!

[roadrunner]

Charlie, nothing rubs off.  The spots are like bumps and are also a bit shiny.  This started on some new plants last winter that had never been outside.  The roses get black spot in late fall and cercospora at times, but they don't resemble this at all.  I have a dozen hibiscus out in the garden this summer and none of them have it.  I just received the Bayer 3 in 1 in an order last week so guess I'll try that.  Our local extension agent hasn't been very helpful for other problems in the past so sending him a sample would probably be a waste of time.

Oh, the leaves are only sprayed a couple of times a week and are always dry long before dark.  A few of my old reliables like Love Story, Blue Bird, and Heaven Scent were nearly defoliated by this and still haven't recovered.  I had to prune them back quite a bit and they haven't put out much new growth and any new leaves that form eventually get the spots again so the plants have remained stunted.

dave

[Darkhorse]

Charlie are leaf diseases incurable like wilt disease?  Can they be spread through human contact?  I would imagine so through pruning, but what about general handling or touching of the plant?

[Charlie]

Good questions, Jordan. Leaf diseases are usually temporary because they are localized infections rather than systemic. Hibiscus have the classic means of ridding themselves of them - dropping the leaves that are effected. Leaf diseases are spread by insects and some can be spread by handling the infected leaves. Leaf diseases can be viral, fungal, or bacterial. The viral ones are mostly spread by insects feeding, another good reason to keep the plants insect free. Fungus and bacteria can also be spread by insects but often they spread via spores in air and water.

Many of these diseases have narrow temperature and humidity ranges that they are active in. That's why I'm surprised that Dave has a problem because he is in the high desert area of Arizona. Hot and dry is not the usual environment for leaf diseases. A commercial hibiscus grower in Florida found that some of his plants got a bacterial disease when left in full sun all the time but did not get it when placed in shade. That was a surprise but just shows how specific the conditions have to be for some of these leaf diseases.

Dave, I'm wondering if this problem could start with physical stress to the leaves. Are the effected plants subjected to a lot of wind?

Charlie

[Darkhorse]

Good info Charlie.  I remember seeing some black spots on a few of my cvs that came from FL.  They're gone now, as I sprayed the plants with Bayer 3 in 1.  Scared me though.

[roadrunner]

Charlie, all the infected plants are in the greenhouse and the only wind they get is from a box fan on the low setting.  I've used this fan the last three summers without this problem.  There is good air circulation in the greenhouse.  Also the afternoon temperature in there usually stays below the mid 90s. 

This problem started in the house during the winter on a couple of new plants.  Those plants were situated in a south facing window.  I recently started briefly showering the plants in the greenhouse in the morning to help battle the spider mites.  Everything dries out reasonably quickly with the box fan going.  I have a door open at one end of the greenhouse and a window open at the other end so their is good ventillation.  Since starting this there has been an increase in the problem.

 Hot and dry is not our typical weather this time of year. That is what we have in May and June.  It is actually quite humid now and it is our rainy season which normally runs from July into September.  Dewpoint today has been in the mid 60s.  Ok you guys in Dallas and Houston, I know that is nothing compared to what you have down there but for us that's humid.  We had thunderstorms last night and just finished with another one a few minutes ago.  Currently we have a 6pm temperature of 72 with 83% humidity.  Our high temp today was briefly up to 89, then the storms started.  Last week we had a few dry days with temps in the upper 90s, but still quite humid.  Still not bad compared to Dallas and Houston.  The weather doesn't appear to be a factor.  I'd expect the problem to be on the outside hibiscus whose foliage will likely remain wet over night from this afternoon's rain as it has several times this summer, yet no problems there. Oh, all those plants are under a small tree and they get morning sun only.

 It looks like about 15 of the greenhouse plants have the leaf spotting.  That's out of a total of 70 plants. Yikes, how did I get so many?  The solution so far has been removing the diseased leaves.  I suppose that now it's time to start experimenting with some fungicides.  I have three: the Bayer 3 in 1 and two from Gardens Alive!, Soap-Shield and Shield All II.

One other thing I'd forgotten about was that I had a chronic leaf spotting problem on a couple of outside landscape hibiscus.  That problem looked nothing like this and started out as small tiny purple spots that grew and increased in number with the leaves finally yellowing and falling off.  I finally gave up on those and got rid of both plants last year.

dave

[Pachrian]

Dave, I googled "Yellowing leaves with black spots" and while I haven't found a difinitive diagnosis, I came across several posts where people suggest spraying the infected leaves with milk. Sounds crazy, but it's easy enough to try.

[Doll]

I found this as it pertained to Hibiscus:
"Besides diseases of the soil, there are fungal diseases that can affect foliage. Leaf spot is the main one to watch for. Container plants may be slightly more susceptible to it than in-ground plants, especially when positioned in places with less air circulation. Dark brown or black spots on leaves indicate the presence of pathogens which are most active in wet weather. Heavily infected leaves, where the individual spots have merged to discolor the entire leaf, will result in shedding, and shrubs so depleted can probably only be salvaged by the application of a fungicidal spray, such as Bordeaux."

[roadrunner]

Uli, thanks for your suggestion.  I always like to try the least toxic options first.  Paul James the gardener guy that used to be on HGTV suggested a diluted milk spray as an eco friendly way to battle black spot and powdery mildew on roses. I think the idea was it would form a protective barrier on the leaves, but I found that as the leaves would grow the protective layer of the milk spray would crack and peel off after a few days so it wasn't effective.

I live in a smaller sized community without any really good garden centers or nurseries so the selection of pesticides, fungicides, and plants for the matter is limited.  I basically have to rely on what Lowe's, Walmart, Ace, and Home Depot stock or mail order which I do frequently.

Doll thanks for the suggestion of Bordeaux.  I've heard of it but can't recall any of the  active ingredient.  Sulphur by chance?  Guess I need to do a little research.  I have two products from Gardens Alive!, the Soap-Shield which I think is copper based and Shield All II which I think is neem based.  My memory isn't always what it used to be.  I'll probably start with those two and see what happens, then go to the Bayer 3 in 1 if necessary and I still have some spider mites to deal with too.  I'll need to get a sample over to our local agricultural agent and hope he has some idea what is going on. With my luck it will be something as rare or unknown in the plant world as my neuromuscular disease last November was to the neurology community.  Meanwhile I'll start with the least toxic options.

dave

[Doll]

Dave, I think you would only use Bordeaux if things had progressed to the point where there no leaves left on the plant.

[roadrunner]

None of the plants have gotten close to the point of defoliation yet. It's mostly a few leaves here and there and strangely on about 15 plants.  Looks like the Bordeaux is copper based as well as the Soap-Shield that I already have so will give the S-Shield a try first.

dave