July 27, 2020 - By Helen Willoughby-Peck, UC Master Gardener, Mariposa - Most foothill gardeners experience problems growing tomatoes at some point. Here are a few that you might encounter:

Fruit Set Failure and Flower Drop:  Plants that don’t set fruit or flowers that drop before fruit set may be due to high temperatures causing pollen to become tacky and non-viable, low light intensity or excess nitrogen which can encourage leaf production but can cause blossom drop.

Keep plants consistently moist and plant in full sun.  Maintain good leaf cover or use a shade cloth during excessive heat. Choose a fertilizer specifically designed for tomatoes. Finally, be patient. Cooler weather will put your tomato plants back into production.

Wilting, yellowing and leaf, branch or plant death: Fusarium wilt and Verticillium wilt are soilborne diseases which destroy the plant’s vascular system, especially in heirlooms which usually aren’t resistant.

Avoid prolonged saturation of the soil and overhead watering which spreads pathogens from infected soil. Remove and destroy infected plants. Next year, rotate plants to uninfected areas of your garden where potatoes, peppers or eggplant (which are susceptible) weren’t planted. Once in the soil, these pathogens live for years. Buy disease resistant seeds or plants. With V, F or VF in the description of the tomato variety.

Sunscald presents as a light brown, leathery spot on the side of the tomato overexposed to the sun. Solar yellowing occurs when tomatoes fail to turn red but stay yellow/orange or red with yellow/orange shoulders. Above 86 degrees, normal pigments fail to form.  Maintaining adequate leaf cover reduces the incidence of these problems.

Blossom End Rot, a brown-black patch on the bottom of the fruit, occurs in green or mature fruit with certain varieties especially prone to it. Inadequate calcium causes the fruit cells to break down, resulting in the leathery spot and subsequent rotting. Soil that is naturally low in calcium, inconsistent watering preventing the plant from taking up calcium, or intense summer heat can be to blame.

Maintain even soil moisture. Apply gypsum at planting time and when blossom end rot appears. Hold out until the summer heat passes and the plant starts producing healthy fruit again.

Fruit cracking:  Large concentric circles of cracked skin are representative of heavy irrigation or rainfall after a dry spell, causing the fruit to swell beyond its skin’s capacity. Large cracks radiating from the stem downward usually are the result of temperatures over 90 degrees or too much sunlight. 

Harvest mature fruit before expected rain to avoid these cracks. Keep soil evenly moist and maintain good leaf cover. Choose plants or seeds that are crack resistant.

For assistance, contact our Helpline at (209) 966-7078 or at mgmariposa@ucdavis.edu. We are currently unable to take samples or meet with you in person but welcome pictures.

The U.C. Master Gardener Helpline is staffed; Thursdays from 2:00 p.m. - 5:00 p.m. 

Serving Mariposa County, including Greeley Hill, Coulterville and Lake Don Pedro
Please contact the helpline, or leave a message by phone at: (209) 966-7078
By email (send photos and questions for researched answers) to: mgmariposa@ucdavis.edu 

For further gardening information and event announcements, please visit: UCMG website: http://cemariposa.ucanr.edu/Master_Gardener
Follow us on Facebook at: https://www.facebook.com/mariposamastergardeners 

Master Gardener Office Location:
UC Cooperative Extension Office,
5009 Fairgrounds Road
Mariposa, CA 95338 

Phone: (209) 966-2417
Email: mgmariposa@ucdavis.edu
Website: http://cemariposa.ucanr.edu/Master_Gardener