Blackening leaves

[1.618]

I would be very interested in Charlies expertise about a problem I am having with my hibs. I had this problem last year also and I think it almost resulted in the loss of some plants before I got it under control.

Last year I noticed that after some heavy rains and putting my plants out in full sun they started to get black areas on the leaves and the plants began to defoliate dramatically. After careful examination and consideration I concluded the problem was most likely fertilizer burn caused in part by acidic rain which resulted in more nutrients being released than the plants could handle. At that time the affected plants had slow release fertilizer mixed into the soil. I guessed that being exposed to full sun magnified the problem. I decided to protect the affected plants from rain and keep them in shade a strategy which seemed to work very well and the plants recovered quickly afterwords. Plants that did not have slow release fert were not affected by the problem even tho they were also out in the rain and in full sun.

I have just noticed that this problem is reappearing in a few of my plants again. This time the affected plants do not have any slow release fert mixed into the soil but the problem has occurred after a rainy period with plants in full sun just like last year. I have been fertilizing with a 24-8-16 at close to full strength roughly every week for the past couple of months. I have put the affected plants in shade and stopped fertilizing them but wondering if I need to stop fertilizing the entire collection? Some pics included below.

[Charlie]

Hi Ian,

I've never seen anything quite like this but from what you describe it sounds similar to a problem that Zoe's Tropicals (now defunct but was in Homestead, FL) experienced and that was diagnosed as Xanthamonas bacterial infection. It was worse on plants in full sun and affected some varieties but not others. They watered by overhead sprinklers at least twice a day during summer and it was this constant wet condition of the leaves that allowed the bacteria to get started and thrive. I've never heard of anyone else getting this type of leaf infection on hibiscus, since most commercial growers use drippers instead of sprinklers and the leaves are not wet all the time.

Zoe's plants that got it also defoliated badly but they would cut them back and grow them again. It became a race between finishing the plant and shipping it and the xanthamonas getting started. They also eliminated the varieties that were susceptible or grew them in more shade since that helped also.

So, since you describe the problem happening "after a rainy period with plants in full sun" and defoliation, that would fit. What I saw on Zoe's plants was smaller spots of black and not wider areas like you show so it may be another type of bacteria or maybe it just behaves differently under different conditions. It's also telling that last year when you protected the plants from rain the problem went away.

I doubt it has much to do with fertilizer, and a lot to do with wet leaves. There is no chemical cure. Zoe's tried some like agricultural ampicillin and Zerotol  (a type of hydrogen peroxide) and they might have helped some but were not a full preventive or cure.

Anyway, this is my best guess. If you want to find out for sure you will have to send leaves to a lab qualified to diagnose plant diseases. I would remove any leaves that show this and try to keep the leaves as dry as possible.

Charlie