Most tomatoes won't set fruit if the nighttime temp is above 70-75F.

Thanks Dan! Our night are in the 60s F. So this may not be the reason...

Then it is your day time temps and/or your humidity or too much
nitrogen fertilizer or inconsistent soil moisture levels or
over-crowding or any number of other stress factors including pests.

Blossom
Drop has several possible causes and the condition has little to do
with a particular variety. There is a FAQ as well as hundreds of past
discussions here all about it.

http://faq.gardenweb.com/discussions/2766768/why-are-the-blooms-on-my-tomato-plant-dying-and-falling-off

http://www.gardenweb.com/gardenweb/query/blossom-drop/topic=tomato

Dave

I think it may be the daytime temperature then..I guess there is noting I can do but wait for the temp to cool down.....in Sep...?!

I don't think it's daytime temperature. The pollen becomes mobile when it's cool. Nothing stopping your flowers from setting fruit in the evening. But over-nitrogen fertilization will do it. Certainly cooler daytime temps would be more optimal, but I don't think high daytime temperatures would stop pollination entirely.

Do you have other tomatoes that are producing that are being treated in the same way? Then the problem is varietal.

Thanks Dan!

I do have other tomato plants, but most of them are cherry types, which are doing well. I also have volunteer medium size varieties setting tomatoes with no problem. The volunteer plants get some shade during the day.

Oh, as with over-N fertilization, I did apply fish fertilizer(5-1-1) to this prudens purple (and to the cherry tomatoes) once or twice in the past month. Could this cause the blossom drop? Should I apply the miracle grow tomato food (10-10-10) or maybe 0-10-10 to help it set fruits?

btw, my ichiban eggplants also have lots of flowers but no fruit. It's my first time to grow large tomatoes and eggplants. So sad.. :(

Hmm. I guess there is something funny about your PP. If you know someone who grows them, that would be who you should ask about fruit set. Both tomatoes and eggplants self-fertilize, so jiggling the flowers when they're open can help them do that.

Sorry to disagree with Dan but high air temps, especially in conjunction with high humidity, is well documented as a cause of blossom drop. It is a chronic problem many of us have to live with - one reason why we try so hard to plant out so we can get fruit set before the high heat arrives.

And once rendered non-viable by the heat and/or humidity it doesn't become viable again in that bloom even when the temperature drops.

When night temps have been below 70 You can compensate for it somewhat by very early morning pollination work on new blooms just opening.

Dave

Thank you all for the info!

For the humidity, I think we have the opposite, it feels like desert in South Cal.. I used an electric toothbrush to pollinate my tomato flowers this morning. Will update if this helps!

Since it seems you live in my vicinity, check for thrips in your blossoms. Right now my tomatoes are getting hammered by them - they get up in there and suck all the juices out, leaving you with dried up brown blossoms that usually drop off at the joint. If you tap a blossom with thrips in it (or vibrate it with an electric toothbrush), you can literally watch them fall/fly out.

For whatever reason they mostly leave my cherry tomatoes alone, and my San Marzanos are still fruiting well, but everything else is struggling. I've been working on Spinosad treatments, which are helping, but it's a thrip plague around here.

As another possibility, and easy to check for.