Diseased sunflowers

Question

My partner went on vacation leaving me in charge of her many plants. Most got some healthy watering from the sky, but I hadn’t realized that the sunflowers are in a pot under an overhanging roof, so they might not have got enough water. The sunflowers appear to diseased.

I looked for similar images on web, but I can’t figure out what’s wrong with them. To me it looks a bit like verticllium wilt but there is not much of a yellow halo. Also our plants have white specks, which doesn’t seem to be characteristic of anything I’ve read about or seen in image searches. There are also some black specks on the bottom. The stalks all look fine. The flowers don’t look infected, but they are sort of shrivelled. Here are some pictures:

Top:

Close up of top:

Bottom:

My questions:

• What does this seem to be?
• Is there a cure or some other action to be taken (e.g., euthanasia)?
• Could it have been caused by underwatering?

Answer 1

It looks like some kind of fungal disease: perhaps anthracnose.

I doubt if underwatering would cause it. However, weak plants can be more susceptible to disease.

Short of pulling up the plants, what usually helps in situations like this is to give them regular fertilizer, keep pests away, prune off affected foliage, avoid over-watering, ensure proper drainage, and take good care of them. Also, pull up all your nearby weeds (they can harbor pests and diseases). If you have an apple tree, don't let apples fall on the ground and sit there to rot (as anthracnose is a common rotting pathogen, and pests may spread it from the apples to other things).

Copper foliar sprays can help with some fungi, I've heard/read.

I might suspect that the plant has mites and/or thrips.

It could be Verticillium, I suppose, considering the drying leaves (especially if they turned yellow before drying), but it's not a very common look for the disease to have round-ish spots in the middle of the leaf.

It might be some kind of mold or mildew.

Although it looks more fungal than viral to me, another possibility is tomato spotted wilt virus, and similar. Such viruses can cause circular necrosis and other weird stuff. They infect many species of plants.