Fungus Lilies And Easter Gnats
(or something similar)
Easter lilies don't suffer from the types of high profile pest problems
of other crops. Not for them the plagues of thrips, whiteflies, leafminer
or mites that can make growers of other crops feel impotent in the face
of the onslaught. Yes, aphids enjoy Easter lilies as much as they like
many other crops, but their attacks are sporadic and usually controlled
without too much difficulty. Another problem can be bulb mites, although
growers often treat for those as a matter of course, early in the crop.
However, the most commonly occurring problems that growers have to face
with their lily crops are usually more difficult to see, and hidden beneath
the soil surface. Not only that, they are often inter-related. Fungus
gnats adults are small black flies, not often seen except on sticky cards,
or when populations get very high. They lay their eggs in the soil and
the worm-like larvae feed on organic matter including roots and root hairs,
and on any soil dwelling fungi. Which leads into the second major problem;
fungal root rots. Diseases such as Pythium root rot and Rhizoctonia can
result in heavy losses for growers, if the crop is not being closely monitored.
And the connection between fungus gnats and root rots? Fungus gnats are
attracted to plants with fungal root diseases, as a particularly suitable
egg-laying site. As the adults fly around the greenhouse, they can pick
up fungal spores and spread them to other plants. The larvae also feed
on fungal spores and can transmit them as they feed or when they become
adults. And the root wounds that are made by fungus gnat larvae as they
feed, make an excellent entry point for new infections.
So what comes first, the fungus gnats or the root rots? It probably doesn't
matter and it's certainly not worth laying awake at night trying to figure
it out. What is important is that both parts of this pest/disease complex
have to be addressed. Drenching a fungicide to control root rots will
not be the best use of your time if a rampant fungus gnat population is
not also controlled. And don't wait until the problem is out of control.
Set up a fungus gnat control program early in the crop and monitor root
growth on a regular basis to detect early signs of disease.
For fungus gnat control, the primary target is the larval population
below the soil surface (adults are notoriously hard to kill) and the issue
needs to be addressed early. Once the crop fills in (by late January or
early February) it can become very difficult to get access to the soil.
The program needs to be started when the bulbs are first planted.
There are a number of options. There are various pesticides that provide
excellent control of fungus gnats. Trumpet as a soil surface spray, or
the insect growth regulators Dimilin or Citation as drenches work very
well. They should be applied about a week after planting (because fungus
gnats love newly planted peat mix) and again if sticky card trapping indicates
the need. The real danger periods to be aware of include:
- the period immediately after first planting
- if bulbs are being pot cooled, then as soon as they are removed from
the cooler
- and just before the crop fills in and it becomes difficult to drench
the growing medium
The other option is the use of biological control which can provide excellent
control of fungus gnats. It is critical to start early (Day 1 preferably)
before fungus gnats populations have a chance to build up. Once that happens
it can be much more difficult for the beneficials to play catch up.
There are a number of beneficial organisms that can be used. Two predators,
the mite Hypoaspis, and the rove beetle Atheta, live in the soil and actively
prey on the eggs and early stages of fungus gnat larvae. Either or both
of these should be applied when the bulbs are first planted. One application
is sufficient. Another useful biological is the nematode Steinernema feltiae
which is sold under a number of different trade names. It is drenched
into the soil in a similar fashion to pesticides. Likewise, the biological
insecticide Vectobac can be applied in the same manner. If you commonly
have fungus gnat problems in Easter lilies, then consider the following
program:
- Don't wait until you see the fungus gnats
- Apply Hypoaspis and/or Atheta when the bulbs are planted
- Drench nematodes about 7-10 days after planting the crop and again
a couple of weeks later. If the crop is to be cooled, apply again when
it is removed from the cooler. A final application could be considered
just before the crop fills in to the point where it becomes difficult
to drench.
- Monitor root health closely for any signs of disease
The last thing you want to be doing is fighting clouds of gnats in the
last few weeks of the crop. Plan early and follow through with your plan.
Figure 1. Fungus gnat larvae (note
the black head capsules) feeding on plant roots.
