What’s causing a problem with our tomato plants? Let’s discuss. – Orange County Register

Q. We are growing tomato plants in pots and have a plant that is not looking good at all. The stems are turning a dark grey color and the leaf veins are turning grey as well. After a while, the leaves turn yellow and fall. The plants looked healthy and were growing well until they weren’t. We have several other tomato plants that are all doing well – only this one plant was affected. What is causing this and is there a cure for it?

When a container plant suddenly collapses, the first possible thing to suspect should be water stress – either an irrigation failure or drainage issues. Growing plants in containers is great if you have limited space and/or crummy soil, but the downside is that you have to be vigilant about irrigation. We’ve lost more than a few plants because of irrigation failure (our cat liked to gnaw on our microsprinkler tubing). 

Your photos showed a very sad-looking tomato plant that has an overall ashy-grey dusty-like coating on the leaves and stems. The fruit looks relatively unscathed, with no obvious lesions or bad spots. This could indicate a bad case of powdery mildew, which is a common fungal infection in our hot, dry climate. Powdery mildew also affects squash and cucumber plants. Sometimes it only affects one plant, other times it will affect neighboring plants of the same type. Once it gets to a certain point, there’s nothing you can do to save the plant. I recommend removing it and discarding in the regular trash (don’t compost it). If you know powdery mildew is a problem in your area, you can use a preventative spray such as Daconil before it appears. You will have to reapply periodically throughout the growing season.

Another possibility is fusarium wilt, or late blight. Fusarium is a soil-borne fungus that affects the vascular system of susceptible plants. The first symptom is wilting on one side of the plant. Soon the rest of the plant will wilt and die. To determine whether fusarium is present, look at a cross-section of stem. If you see a ring of brown or grey diseased tissue (this is the dead vascular system), chances are your plant is infected with fusarium. Once it’s infected, it’s a goner and should be removed and disposed of. If you suspect that fusarium is lurking in your soil, the best way to avoid future heartbreak is to select tomato varieties that are resistant. Look for the F or FF on the plant label, which indicates that variety is resistant.

Los Angeles County

mglosangeleshelpline@ucdavis.edu; 626-586-1988; http://celosangeles.ucanr.edu/UC_Master_Gardener_Program/

Orange County

ucceocmghotline@ucanr.edu; http://mgorange.ucanr.edu/

Riverside County

anrmgriverside@ucanr.edu; https://ucanr.edu/sites/RiversideMG/

San Bernardino County

mgsanbern@ucanr.edu; 909-387-2182; http://mgsb.ucanr.edu