A Yes, it does look like a disease problem but I think it will stop spreading as soon as the humidity goes back down and rains stop. In fact, this problem most likely occurred a few weeks ago since the new leaves coming out seem unaffected.
|Plant disease triangle needed for disease to develop|
. Diseases need to have three ingredients, all coming together at the same time, to manifest themselves. First, the plant needs to be susceptible to the disease. Plants that are not susceptible or have been bred to be resistant to a disease will not normally get infected.
Secondly, the disease itself needs to be present. This is why I continually ask readers to sanitize their equipment, inspect plants for problems before you buy them and rotate your vegetables.
The third ingredient is the right kind of weather. Most diseases are caused by fungi, the majority of our plant diseases here, and grow and spread under high humidity and rainy weather.
Disease control focuses on interrupting or preventing one of these three ingredients. All we need is a drop in the humidity and it will stop. Also pruning so that you have good air movement through the canopy will also help prevent and reduce its spread.