Hello Katya,

Thanks for reaching out and sorry to hear about this alarming situation!

It's a little hard to tell from the photos, but one likely culprit is that your balcony is getting too hot and/or the plants are not getting enough water. If that corner of your balcony gets lots of direct sun or gets very hot in the afternoon, you may consider moving them to a more protected space. If the soil looks dry in the pots, you may need to water more regularly. (I water the plants on my south-facing porch daily when it is hot and sunny).

If you think you have ruled out heat or under-watering issues, the problem could be powdery mildew, a very common fungal infestation that arises amid heat and humidity. Given our area's recent warmth and rainfall, this is certainly a possibility. Not to worry, powdery mildew is not usually fatal to plants and can be managed by promptly removing infected leaves and stems. Keep a close eye on your plants throughout the growing season and monitor for symptoms. Powdery mildew likes moisture, so cut back your watering if you think you are over-watering. You can also try watering from above so that the water falls over the leaves, rather than watering directly at the roots. Chemical treatments should not be necessary to control this problem. Below is a fact sheet on powdery mildew from Purdue University, but there are many other reliable resources from universities and Agricultural Extension programs online.

https://www.extension.purdue.edu/extmedia/bp/bp-5-w.pdf

Please don't hesitate to reach out again with more questions or photos. Good luck!

Best,

Andrew

Yes, DC Master Gardeners have a table staffed by volunteers at the  Saturday morning Farmers’ Market at Van Ness, convenient to the Van Ness Metro stop.  Great idea to bring fresh samples, don’t add water to the bag, in a plastic bag, to see if they can identify the cause(s).

Adding to Andrew’s concern of  Powdery Mildew, do you see aphids on the leaves? They can cause similar symptoms and can be comorbid. The picture of leaf undersides that is enlarged appears to have aphids present.  Aphids are very common, spread quickly, do little damage in small numbers, but large populations can seriously affect, even kill, plants. 

Fortunately, there are simple remedies.
First, remove any dead leaves & stems, cut stems down as needed, discard any dead plants and  brush the soil surface gently.
Second, before using a spray bottle of water with a somewhat robust spray (spray not jet), cover the soil (paper, cloth, etc.) just so it doesn’t spill while you tilt the plants.
Third, tilt the plants and completely spray the undersides of all the leaves with water. If any add’l dead leaves fall off, just discard those leaves. Plain water is surprisingly effective on adults, but the addition of a very small amount of soap could help deal with the young. Only recommend pure soap such as Dr Bronner’s Castile liquid soap, unscented is fine.  (Dawn, for example, is detergent and now contains grease cutting boosters that could potentially be harmful to plant cuticles.) Use of insecticidal soaps isn’t necessary for soft-bodied insects like aphids. The castile soap might also impede powdery mildew somewhat, if present. 
Fourth, wipe off plant stands and other surfaces the plants are on to remove any aphids present.

Repeat spraying every couple of days to wash off subsequent generations. Then, move to weekly spraying until new growth is established. Taper off until no longer needed.

Please let us know how if works for you, feel free to stay in touch and to consult with DCMGs in person at the market table; we are here to help.

Kate