Farm Calls: A Greenhouse “Walk of Doom”

Tomato transplants. Photo: P.Nitzsche
Last month at a growers meeting, a farmer remarked how calling his county agent out to the farm, more often than not, turned out to be a “walk of doom” because the agent always found problems he didn’t even know he had. The farmer is actually happy about this – it means he gets the jump on problems before they get out of control. This week’s farm call from a south Jersey grower reminds us why there’s no substitute for agents in the field.

Can you come take a look at my tomatoes?
They’re still in the greenhouse due to the cold temperatures and I’m seeing some wilt. The roots are turning light brown with the outer sheath sliding off.

The Diagnosis

With winter temperatures continuing into early spring, most plants are still in the greenhouse. The longer plants stay inside, the more issues can arise with growth and pests. The plants had already set some golf ball sized fruit and they were now under stress due to the energy and nutrients moving into enlarging fruit. The roots were indeed light brown and the outer sheath easily slid off. With a degrading root system, the roots could not keep up with the water demand of the top growth. Pythium root rot was the cause of the wilt symptoms. Pythium fungal spores can be found anywhere soil, water, and plant debris exist. We recommended a fungicide drench. Problem solved.

Oh, by the way…

On our way out of the greenhouse, we recognized fungus gnats flying around the base of some plants and the media mix. Moving a hand across the surface of the mix flushed up more. Most times fungus gnats are not a worry in vegetable transplant greenhouses where plants stay for a couple of months and then go out into the field where the environment is not conducive to fungus gnat survival. However, research shows that fungus gnats, like shore flies, can be efficient vectors of plant pathogens (El-Hamalawi and Stanghellini).

Could the tomato Pythium root rot be linked to the fungus gnats in the greenhouse? Leanne Pundt (UConn) states, “Pythium fungus can provide a complete nutritional food source for the fungus gnats to develop from egg to adult. Adults may help spread the disease by carrying Pythium oospores on their bodies as they move from plant to plant”.

Fungus Gnat Detection, Description, and Treatment Options

Detection: Close inspection of plant base and media, and use of yellow sticky cards placed horizontally.
Description: Fungus gnats are mosquito-like: small (less than 1/10 of an inch long), slender, dark charcoal in color, and have long legs. Antennae have many segments and wings are delicate with a “Y” shaped vein near the wing tip. To distinguish fungus gnats from shore fly adults, check for these differences: shore flies have stockier bodies, shorter legs and shorter antennae, with wings that have distinctive faint light spots. Identification is important since management strategies differ for the two pests. Both pests can rapidly increase in population if left unchecked.

Fungus Gnats Yellow Sticky

Sticky card with fungus gnats. Photo: S.Rettke

Fungus gnat

Fungus gnat.
Photo: L.Pundt

Fungus gnat life cycle.

Fungus gnat life cycle. Figure: G.Ghidui


Treatment Options: The first line of defense against fungus gnats is cleanliness both in and out of the greenhouse. Eliminate rotting vegetation, weeds and grasses near the greenhouse. Clean out gutters, downspouts and other areas that support algal growth. Algae in a damp environment provides an ideal location for fungus gnat larval development.

There are insecticide treatments and biological controls available for management of fungus gnats; see NJ Commercial Vegetable Production Recommendations and check labels for crops and specific growing situations.


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