With the spring season going full tilt in many areas of the country everyone is busy growing, shipping and taking care of customers. Typically, with so many flowering plants in the greenhouse, insects and diseases tend to sneak up on many growers. The warmer weather speeds up insect life cycles, making a small population into an epidemic within a short time. Oftentimes, thrips, mites and aphids don’t show up until you are shipping or selling product. With rainy weather, even the best spray programs have trouble keeping up with Botrytis on old flowers. Powdery and downy mildews can sneak up on susceptible crops with close spacings and the right weather conditions. So, let’s focus on some key insects and diseases and how to control them.
My top four diseases to worry about are Botrytis, powdery mildew, downy mildew and Rhizoctonia. Botrytis likes dead and dying tissues (such as old flowers and lower leaves), needs 4-8 hours of moisture to infect and creates a huge number of spores. When cleaning plants, you distribute those spores all over the place, where they can land on open wounds or just sit on the tissue waiting for water to infect. The first line of defense against Botrytis is proper moisture management. Get your watering done in the morning and dry off leaves going into night. Use a venting or dehumidification cycle in the morning and evening to get the humid air out of the greenhouse. Good air movement within the greenhouse also helps. Remove dead leaves and flowers, and keep the greenhouse clean. The best chemical controls include Daconil, Chipco 26019, Decree, Medallion, Milstop and Phyton 27. For safety on flowers and less residue, rotate Decree and Medallion (not on geraniums).
Powdery mildew first shows up as white spots on leaf tops. Certain crops, such as verbena, zinnia, rose and tuberous begonia, are very susceptible to this disease and should be used as indicators. This disease does not need free moisture but likes high humidity and cool nights. Use a preventative spray rotation every two weeks of Compass (or Heritage) and Systhane (or Terraguard or Strike). Phyton 27, Milstop and Rhapsody are also good.
Downy mildew symptoms are sometimes difficult to see before you have major problems. Grayish lesions show up on leaf undersides then progress to the upper leaves. Huge amounts of spores are produced in the morning, and high humidity and moisture are needed. Susceptible crops include alyssum, stock, snaps, pansy, rose and many perennials. Spray every week with Heritage, Stature DM or Aliette. You can also use Dithane or Protect, but you must get under-leaf coverage to be effective.
Rhizoctonia sneaks up on susceptible crops at close spacings and wet weather or high humidity. Symptoms typically show on stems at the soil surface as black lesions or a wire-stem. Plants collapse and turn black. Good air movement, proper moisture management and venting will help control this disease. You can drench monthly with Cleary’s 3336 (or OHP 6672 or Fungo), Medallion (not on impatiens or geraniums) or Heritage. Or you can thoroughly sprench with Chipco 26019 or Daconil.
My top four insect problems are thrips, aphids, whiteflies and mites. Thrips can damage leaves and flowers, rendering plants unsaleable. In addition, they transmit impatiens necrotic spot virus and tomato spotted wilt virus, which have a wide host range and also render plants unsaleable. These viruses can show different symptoms on different crops, and some plants may not show symptoms at all (Typhoid Marys). Use yellow or blue sticky cards to monitor thrips. Cards should be placed 2 inches above the canopy, checked regularly and changed weekly. Also, place cards in basket lines, as this is where thrips will be found first, due to warmer temperatures. Use one card per 1,000 feet2. Use a rotation of chemicals for control, but start before thrips get out of hand. Best chemicals include Conserve, Avid + Talstar (or Ornazin), Mesurol, Thiodan, Pedestal and Botanigard. Except for Pedestal, apply each chemical two times before rotating to a different chemical. If the thrips population increases, increase applications to every 3-4 days instead of weekly.
Aphids always seem to show up overnight on crops that are ready to ship. In fact, this insect can reproduce rapidly (it doesn’t need a sex partner!) and loves certain crops, such as pepper, celosia, fuchsia and asparagus springeri. Typically, you will find aphids in clusters, not throughout the greenhouse. Scouting is your best bet for prevention. On crops requiring longer grow times, use a drench of Marathon or Flagship when roots are to the sides of containers. For a spray rotation, use Endeavor, Marathon II (or Flagship, Tri-Star or Safari) and Orthene.
On most bedding plants, whiteflies are not a problem. But with all of the vegetative annuals now being grown, along with bigger baskets and containers, this pest will be there. Sticky cards may show you some whiteflies, but you need to scout plants by flipping over or brushing leaves to see the insects. Susceptible crops include verbena, fuchsia, bacopa, gerbera, hibiscus and lantana. Again, use a drench of Marathon (or Flagship) when roots are to the sides of containers for 8-10 weeks of control. Otherwise, use a spray rotation of Endeavor, Marathon II (or Flagship, Tri-Star or Safari) and Botaniguard. Distance, Talus and Pedestal are great insect growth regulators that should only be used once a crop.
Finally, mites. Generally, we are concerned with two-spotted spider mites, although broad and cyclamen mites can also be problems on certain crops. Spider mites love hot, dry weather. Look for mite problems on crops hanging up or in warm greenhouses. For scouting, look under lower leaves with a magnifier. If you wait to see webbing, you might as well throw out those plants. Stippling and striping of leaves are other signs of mite damage. Use a spray rotation every 2-3 weeks of Avid (or Tetrasan), Akari (or Sanmite), Floramite or Pylon. If populations get high, you will need to combine the above with Hexygon (or Ovation) to control eggs. With most of these chemicals, under-leaf coverage is most important, so spray thoroughly.
There you have it — my spring pest checklist! Get out into those houses and fields and see if you are controlling them or not. With everything that is going on now, controlling pests and diseases should be at the top of the list, not an afterthought.