Expert: C.L. Fornari - 7/5/2008

Question
I live in Denver, and we have had lots of 90 degree weather.  I have a hanging basket of petunias.  I just noticed that it has little tiny black spots all over the ends of the plant.  The entire plant is covered so I was thinking that it is more likely to be a fungus than a pest.  If it is root rot, what should I do to care for it?  My pot does not have holes in the bottom.  Should I poke holes in the bottom?  How often should I water it?  It dries out very quickly here.  Thanks

Answer
Rachel,
Have you tried touching the black spots? Do they crush under your fingers?  If so, they are black aphids - common on the new growth (ends) of annuals and easy to control either by crushing with hands, insecticidal soap or a hard stream of water.

If the spots can't be crushed it may be leaf-spot fungi - leaf spot happens under moist conditions, and is often caused by splashing of foliage with water on a frequent basis. Leaf-spot seldom kills a plant but is cosmetic. At this point if it was my plant I'd do the following:

1. Yes - poke drainage holes in the bottom of the pot. This may not be related to your spots but is helpful nonetheless.
2. If the black spots can be crushed spray with insecticidal soap.
3. If they can't be crushed, cut the tips of the plant off so you're removing the black spots - this will make the plant more full and bushy anyway, and you might cut some of the branches way back to 4 inches above the basket as well - a good practice with petunias always because they bloom only on the ends of the branches - pruning a few back every other week or so is how you can keep a petunia basket filled with flowers all summer.  Throw the tips that have black spots out.
4. When you water, water in the morning and try not to get the foliage wet.
5. Fertilize after watering - never fertilize a thirsty plant - every three or four weeks or put a time-release product on now that will feed through the end of the summer.

I hope this helps!
C.L.