Hi Tom. I grow tithonia and this has happened to me more than once. It doesn't seem to hurt the plant and is related to a period of when they go too dry. It can be quite unsightly, but have found when conditions go back to normal, they'll develop new leaves at the axils and end up being (and looking) just fine. Once they are in ground for a good while and established, they seem immune to it happening and tolerate intense heat and sun well.

Thanks, that is exactly what is happening. I was worried because last year they were beautiful with no problems and I couldn't wait to plant them again. I have been fighting slugs all spring and I think the brown leaves were caused by them or at least I am blaming them.

Slugs will cause holes in leaves, but not make them brown. I grow them commercially and if I mess up and let them get too dry in the seedling stages, I can take it to the bank the leaves get nasty. They snap out of it, but I loose a few weeks' sales.

Should I cut the brown leaves off or let them go?

I usually pick them off, since they don't seem to shed very easily and dead leaves, if the weather suddenly becomes damp for a long period, can cause some rot problems, but it's more a matter of how much "ugly" one can stand. LOL. I love tithonia and use them in a particularly nasty bed with harsh exposure and they always do well.

Hi Calliope,
Came home this afternoon and now the tips of the new leaves on the tithonia are starting to turn brown. I am confidant now that I won't have any of my favorite flowers this year.
The zinnias have recovered from the slug attack and are starting to form flowers. Also this is my first year with nicotiana and they are starting to bloom. Do you know anything about cleome? The very bottom leaves are yellowing on them. The weather here has gotten unseasonably hot after a cold, wet spring.

I started mexican sunflowers indoors and transplanted them in mid March here in San Antonio TX. By May I had flowers and now they are almost 7 ft tall w tons of flowers. Suddenly in mid July plants look like they are dying! Leaves turning brown and plants don't look like they'll survive. It's been very hot here. 107 today and it's been over 100 for at least a week. They are supposed to be drought tolerant so although we've been giving them some water, I'm wondering if they need more. Do you think they'll rebound or should I start new plants. I had anticipated having these through November. Any advice thanks

In my experience tithonia need regular watering. They have huge root systems but they also have large leaves so they lose a lot of moisture on hot days.

Deanna