May 1, 1998
Compiled by Vern Grubinger, University of Vermont Extension
(802) 257-7967

Here's a summary of information presented at the Greenhouse Tomato Roundtable by Gillian Ferguson, Pest Management Specialist with Ag Canada, Ontario. (Note: suppliers of greenhouse predators and parasites were listed on the vegetable and berry page in the March 15 Agriview)

Biological control of aphids using the tiny wasp Aphidius requires proper identification of the aphid species you've got. Green peach aphid, probably the most common aphid on tomatoes, will be parasitized by Aphidius colemani, but potato aphid, found increasingly in Vermont greenhouses, will not. For potato aphid, Aphidius ervi and Aphelinus abdominalis will work. Body color is not useful for distinguishing among aphid species. Compared to green peach aphid, the potato aphid is larger, more oval shaped with a longer 'tail', has longer, spider-like legs, and drops from the plant readily when disturbed. You can tell if Aphidius is working because after they lay their eggs in an aphid nymph the result is a blackened 'mummy'. Another biocontrol is Aphidoletes aphidimyza, a small black fly with a predaceous larval stage that bites aphids on the knees, paralyzes them, then sucks their guts out. Who says biocontrol isn't fun? Aphidoletes will attack most aphid species, nymphs and winged adults, but is best used to control 'hot spots' of aphids since it will kill more than it needs to feed on. To sum it up, here are aphid biocontol options: Green peach aphid: Aphidius matricariae, A. colemani, Aphidoletes aphidimyza. Cotton aphid: A. colemani, A. aphidimyza. Potato aphid: A. ervi, A. aphidimyza or Aphelinus abdominalis.

Biological control of 2-spotted spider mites using the 'tomato strain' of the predator mite Phytoseiulus persimilus can be effective, but they do not like to eat the overwintering population of 2-spots, which have an orange coloration and look a lot like the predators. Amblyseius fallacis or A. californicus can be released early in the season until the normal looking generations of 2-spotted mite appear. However, the effectiveness of the Amblyseius species on tomatoes is not certain. To be effective, persimilus needs at least 65% relative humidity and warmth. It should be throughly distributed by placing directly on the leaves throughout the canopy wherever mites are present. Besides P. persimilis, othr spider mite biocontrols are Feltiella acarisuga or Therodiplosis persicae (predatory midge) and Stethorus punctillum (ladybeetle) the latter two are not very effective on tomatoes due to hairs.

Biological control of whiteflies with Encarsia formosa can be enhanced by thorough dispersal in the greenhouse. Under cool, low light growing conditions, request cards from your supplier with as few as 35 parasitized pupae rather than the conventional 70-100 pupae per card. That makes for more points of release and therefore more even distribution throughout the crop. Place the cards one-third of the way down the plant, protected from sunlight. Maintain a minimum of 70 F daytime, and even higher temperatures will promote dispersal and reproduction of Encarsia. Do not remove older tomato leaves that may have parasitized whitefly larvae on them or, leave removed leaves on the ground for a couple of days to allow for parasite emergence. However, remove any diseased leaf material immediately out of the greenhouse regardless of presence of parasites. A novel but tedious method of quickly reducing adult whitefly populations in hotspots can be carried out by "vacuuming" the uppermost leaves. Besides E. formosa, other whitefly biocontrols are Eretmocerus californicus (warmer weather) and Delphastus pusillus (for hot spots, not usually effective on tomatoes)

The small fruit discussion list has recently featured exchanges about the effects of tipping back red raspberries. The consensus: tipping is advisable, as it will increase fruit size and if done in moderation, will not have a dramatic effect on total yield, i.e. fewer, larger fruit will result. Aesthetics of the planting will also be improved. Remove 10 to 25% of cane length.

PREVENTING LEGGY SEEDLINGS is a good idea because tall spindly seedlings have a poor survival rate upon transplanting. Besides low light conditions, high day temperatures and low night temperatures are one factor that leads to leggy seedlings. If day and night temperatures are equal, seedlings will grow to be more sturdy. Since it is difficult to make day/night temperatures equal, one solution is to lower daytime temperatures for a two hour period starting just before dawn by ventilating when it is still cool outside. Low light conditions can be caused by not just by cloudiness but also by failure to change the plastic on the greenhouse in a timely fashion. As plastic ages, the amount of photosynthetically active radiation passing through it declines. Dirt, scratches, condensation in and on the plastic also limit light getting to the plants. Avoid fertilizing transplants during long cloudy periods. Over watering can also cause excessive transplant growth. It is best to let the soil surface dry out before watering again.

Seedling stems will be shortened and strengthened in response to movement. Some growers use fans to gently move seedlings from side to side. Brushing plants has also been shown to reduce legginess and improve stem strength in crops like tomato. Wait until the plants are about 3 inches high so the growing point is protected by leaves. Use an unpainted broom handle (so the leaves don't stick to it), and brush back and forth 10 times per day, any time of day, at any speed. If you like, wear a black pointed hat and squeal 'I'll get you, my pretty!."

Once again, problems are occurring with irregular seedling growth of lettuce and tomato in a compost-based potting mix. Older roots appear brownish with few feeder roots, but no lesions. Healthy young roots are visible. The cause appears to be an initially high soluble salt content, and the crops are growing out of it after sufficient watering to leach out the salts.

Spider mites are appearing in greenhouses in Vermont and elsewhere. Appears to be more of a problem where some vegetation was present over the winter rather than clean fallowing. Get out your hand lens and examine leaf undersides - sooner rather than later.

Thrips are a problem in a number of greenhouses, from Shaftsbury to E. Hartland. Mostly on ornamentals so far but keep an eye on adjacent vegetables! Problem occurs most often where ornamentals are purchased in - ivy geranium is one popular culprit. Control with labeled pesticides - biocontrols an option but may not work fast enough to maintain foliar quality on bedding plants. Plan to clear house of vegetation and heat up to 100 degrees F for 3 consecutive days at the end of the season to promote thrips emergence, starvation and death. Biocontrols: include: Amblyseius cucumeris, A. degenerans or Orius insidiosus

Abiotic disorders diagnosed on tomato seedlings in the greenhouse: low temperature injury (whitish areas of dead tissue on older leaves). Oedema - blistering of leaves on lower surfaces, caused by a combination of warm soils and poor ventilation (high humidity). The roots keep pumping moisture up to the leaves, but transpiration is shut down, so the cells in the leaf essential 'explode'. Solution: heat up air the turn on ventilation fans or roll up sides, even if it's cold out, then re-heat house.

Strawberries are putting on new growth, and garlic is up and looks good around the region - peas, corn, direct seeded brassicas, lettuce, onion transplants going in at some locations. Appears we are about 1 week ahead of "normal" so far.