Issue: October 13th, 1997

Squash leaves turned black

Some of the leaves on my squash plant turned black this morning. Last night the temperature dropped to the mid-thirties, but it didn't freeze. I have a recording thermometer and checked. Did my squash get a disease last night? Will it spread? Is it okay to compost the plants?

The leaves were blackened by a light frost. Even though the thermometer didn't record a freezing temperature at its location, the location of the plants in the garden was probably just at freezing. We tend to put our thermometers at eye level, several feet above the level of plants in the garden. On a calm night, temperatures at the surface of the leaves can be several degrees colder than at thermometer level. Leaves which were injured by frost will blacken as they are heated by the sun in the morning. Upper surfaces of leaves that are exposed to the night sky will be most subject to radiative cooling and will be the ones injured. Leaves covered by other leaves will retain a little more heat and not suffer the frost damage. You didn't notice the frost itself because it was spotty and quickly dissipated as the sun rose. So, your plants didn't get a disease for you to worry about.

The plants may, and should, be composted. If only a few leaves were injured, the plants may continue to grow and mature fruit for another week or several weeks until another cold night freezes or allows a more extensive frost.

Fruit and squashes on the plant which were protected from radiative cooling may still be left on the plant. Any fruit that shows damage due to the frost should be discarded. Be sure to harvest any mature squash before the next cold spell.

Frozen tomato safe to eat?

My tomato plants froze pretty severely a couple of nights ago. I just pulled them up and threw them away. A friend told me I could have saved the tomato fruit and eaten them. Is that right? I thought they were ruined by being frozen.

If the fruit were not frozen and after a couple of days had shown no symptoms of injury, they could have been harvested and eaten. However, if they were injured by frost, then disposal is the appropriate treatment. According to Patricia Aaron, Home Economist for the Bernalillo County Extension Service, it is best to err on the side of caution. Any fruit damaged by freezing begins to decompose, or rot, rapidly as it thaws. Since there are fungi and other decomposing organisms which produce chemicals toxic to humans and pets, if the fruit was damaged, it should be discarded.

According to Patricia, many people take unnecessary risks with unsafe food. They think they can just cut off the affected part of the fruit, and then it is okay. She points out that a fungus can extend further into the fruit than the damage. Though we may not see the fungus, it may penetrate far into the fruit. If the fungus produces a toxin, then by only cutting out the damage they see, these people ingest the poison endangering themselves, their family, and anyone who shares the meal with them.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!