Are bigger/older hibiscus able to fend off disease better than younger ones?

[Bradley]

I have a situation in my back yard in which my smaller hibiscus appear to be defoliating due to a fungus while my larger ones are not. All variables are the same: location, new terra cotta pots, soil, water, sanitized clippers, & fertilizer, with the exception of size. Are more mature hibiscus able to fend off disease better than younger smaller ones?

[Charlie]

Bradley, why do you think it is a fungus causing defoliation? That would be unusual for hibiscus.

Your general theory is pretty much true - older hibiscus are usually woodier and tougher and have stronger defenses than plants just getting started.

However, there are other issues - water and fertilizer and insects being the first to look at. Most defoliation of hibiscus is due to spider mites. Second is water or soil issues which are often related. Its very hard to diagnose from a distance but I would have to put leaf fungus attacks close to the bottom of the possibilities.

Charlie

[Bradley]

Charlie,

   I just assumed that fungus was what was defoliating the plant based on what I was seeing. The leaves get these black spots after heavy rains, and then the area around the black spots turn yellow. This is what heavy rain is in Florida: On July 11th we had 1.77 inches of rain, .76 on the 12th, .4 on the 14th, .48 on the 15th, and 1.31 inches on the 16th. Because the black spots are all over the leaf, the yellow takes over and the leaf is done. Only the leaves that get these black spots turn yellow. I've attached pictures of what the leaves look like after a big rain on my small hibiscus, please tell me what you think. These pics are from a similar heavy rain period in May. All my big hibiscus get a few black spots after a big rain, but shrug it off, my small ones don't.

    Anyhow, I follow all of this website's hibiscus care suggestions, so I didn't think it was other issues. My soil is Fox Farm Ocean Blend, fertilizer is 1/2 tsp of HVH per gallon daily, all have new terra cotta pots with extra drainage holes, water currently is 2x daily adjusted for rain (7am and 4pm / fertilizer in pm), and all have been treated with Bayer insecticide and now several different contact and systemic fungicides. Once per week I give them Super Nova and occasionally Super Thrive per label instructions. I also assumed it was fungus because the 2 climbing roses I have that are near the hibiscus have gotten black spots and also defoliated.

These 2 facts perplex me:
1. None of my big hibiscus have this issue (survival of the fittest?)
2. If the rain stops or slows, which it has this past week (.25 inches all week), the black spots stop multiplying and the yellowing of the black spots stops.