Although native bees and honeybees may just be starting to gather strength and are beginning to fly outside, other “B’s” have been of growing concern in the greenhouse for some time now.
These include common spring bedding crop problems like Botrytis cinera (aka grey mold), Broad mites, and leaf burn (from a variety of causes).
Keep reading for tips on how to manage these issues during this time of year.
Broad Mite (Polyphagotarsonemus latus):
This is the time of year when Broad Mite populations start to build and become noticeable. They like to attack crops like New Guinea impatiens, torenia, begonias, exacum, ipomea, mandivilla and gerbera.
Given their small size (0.2 mm), spotting this mite is near impossible with just a hand lens. Damage is usually the first sign, so susceptible crops should be walked and inspected regularly. Damage can occur at very low mite densities, and often looks like this:
Once found, Broad mite can be effectively treated with biological control. I’ve seen high levels of N. cucumeris (applied as a weekly broadcast) do an excellent job of surpressing this pest. Other mites species can also work. For more details on broad mite biocontrol see these articles from Michigan State Extension and Dr. Rose Buitenhuis. Note that the damage caused by broad mite feeding may take a few weeks to disappear once they’re under control.
For broad mite, there are few effective miticides. Both Avid (abamectin) and Forbid (spiromesifen) have translaminar activity, meaning complete contact with all mites isn’t necessary (yay). However, BOTH chemicals are also toxic to predatory mites (boo). Forbid is also tricky when it comes to phytotoxicity (always read the label). So, unless you’ve got a major problem going, these products may be best used for hot spots or as clean-up applications before shipping.
Botrytis cinera (AKA Grey Mold):
One of the most common and destructive diseases of greenhouse crops, outbreaks usually follow periods of cool, damp, cloudy weather. Unfortunately, Botrytis cinerea has an extremely wide host range. Flowers with thick succulent petals, such as begonias, peonies and geraniums, are particularly susceptible. The disease also commonly affects African violet, dahlias, pansy, snapdragon, zinnia, chrysanthemum, and others.
Botrytis can cause different symptoms on different plants, so you may need to confirm your diagnosis by sending samples to Guelph Lab Services.
However, Botrytis infection usually begins as brown to gray circular spots that appear “water-soaked“. Later, tan to gray fuzzy mold develops on rotted tissue under humid conditions.
Botrytis thrives on humidity: maintain humidity below 85 percent by (a) forced circulation or (b) an increased amount of heat. Proper bench spacing is also essential in reducing canopy humidity.
Unfortunately, most chemicals for Botrytis control are preventative. They’ll help control the spread of the disease, but are unlikely to cure it once it happens (which is why cultural controls, like air circulation, are so important here). If you do apply chemicals, remember to rotate chemical classes, as Botrytis is highly adept at developing resistance.
For a list of effective products, see my previous post on Botrytis here.
Types of Burn:
We’ll stick to environmental “burn” in this post and leave an application damage discussion for another time. The climate inside the greenhouse in the spring can be less than idea for plant growth – especially if we get a stretch of grey rainy days. This causes lots of issues that can be mistaken for diseases, pests or even viruses. Generally speaking, growers often report symptoms like stunting, leaf cupping or distortion and burn along the leaf edges.
Humidity issues: The weather conditions outside and the humidity inside the greenhouse in the spring can be a recipe for disaster. When the relative humidity in the greenhouse is high, plants have a hard time transpiring. Their roots actively take up water, but their leaves cannot release it since the air around the plant is already very humid. Humidity is related to temperature, so this can also happen when the media the plant is grown in is warm and wet, and the air surrounding the plant is cooler. In the case of oedema, water pools in leaf tissue until cells become malformed or the area around them collapses leaving a necrotic spot which can look like burn on the top or underside of leaves. Look for symptoms across a wide area of plants, more information can be found in previous blogs posts here.
Improperly vented heaters: Spring brings warmer days with cool nights, meaning the heat is still on in the greenhouse. If you use heating units in the greenhouse that are not properly vented, you run the risk of burn from impurities such as carbon monoxide (CO), sulfur dioxide (SO2) and ethylene (C2H4) . Leaf curling and flower drop can be symptoms of ethylene damage, while leaf edge burn and chlorotic spots are symptoms of sulfur dioxide damage. For more information on how to detect this damage and prevent it from happening again, see our previous blog posts on this topic here.
One time issues (aka something went wrong and I don’t know what it was): This type of burn can be frustrating because you can do everything right and it still might happen! Damage causing “events”, usually are noticed after the fact, and generally show up on leaves of the same age. For example, the damage might look worst on leaves that were just developing a few weeks ago and be less apparent on new growth. In order to help solve mysteries of this kind, it’s a good idea to keep track of issues (even small ones) as they can compound resulting in the damage. You may not be able to prevent this, but understanding what happened is a good reminder for what conditions to avoid in seasons to come. Sometimes damage from one-time events like humidity issues, cold stress or fertilizer burn can be overcome as the plant fills out and covers the damage on older growth, so consider the options available to you before jumping into panic mode.