Humidity causes most plant disease problems

Question: I am sending you a picture of my funky-looking tree. I think it has some sort of disease problem.

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 probably occurred a few weeks ago, because the new leaves coming out seem unaffected.

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 normally will not get infected.

Second, the disease itself needs to be present. This is why I continually ask readers to sanitize their equipment, inspect plants for problems before they buy them and rotate their 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 help prevent and reduce its spread.

Bob Morris is a horticulture expert living in Las Vegas and professor emeritus for the University of Nevada. Visit his blog at

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