6 diseases to watch for in spring

As winter turns into spring throughout most of the country, gray mold (Botrytis) is slowly retreating and other diseases are advancing. Ornamental producers are always in advance of a plant’s natural season to have product in time for the consumer’s planting season. Plants grown out of their optimal range of temperature, daylength and humidity can be stressed and weakened resulting in more disease problems than they would normally experience. Other diseases only occur in spring since that is the only time when the new plant growth coincides with temperatures that favor the pathogen.

Theses are the major six diseases to be on the lookout for.

1. Botrytis

Botrytis blight is very common on dahlia, fuchsia, geranium, cyclamen, exacum, poinsettia, pansy and lisianthus. All ornamentals can be affected by this non-discriminating fungus. Thus, in spring when Botrytis cinerea would be expected to be less severe, or hopefully gone, it can continue to be a problem on summer crops. These crops are optimally produced under warmer conditions. A cool spring may trigger Botrytis infections in crops like salvia, vinca and zinnia.

What to watch for. Watch for tip blights and stem rot. If you cannot use heating and venting to reduce humidity over night, a fungicide may still be needed.

Right about the time you think that Botrytis outbreaks over, start looking for Sclerotinia. This fungus causes severe crown rot and blight on many bedding plants, especially those grown in massive outdoor beds.

Controls. Our research trials show that Botrytis is best controlled with one of four fungicides: iprodione (Chipco 26019, Chipco 26GT and Sextant), chlorothalonil (Daconil formulations), fenhexamid (Decree) and fludioxinil (Medallion). There are a number of experimental products with a high degree of activity against Botrytis with new active ingredients or combinations of active ingredients.

Read the chemical labels carefully. Use all products according to their labels only.

2. Bacterial leaf spots

One of the most common problems to watch for during spring is bacterial leaf spots. They can pop up almost overnight on crops including Erysimum, lavender and impatiens.

What to watch for. If you have not seen any bacterial diseases this past winter, consider yourself lucky. Spots and blights caused by Pseudomonas and Xanthomonas are most severe under moderate temperatures but do require free water on leaves to spread and infect.

In winter, it may be too cool for optimal bacterial development. The heat of the greenhouse may dry leaves too quickly for the pathogens to express themselves fully. In spring, it will be warmer, heater use will decrease and the greenhouse humidity may increase. Bacteria also tend to grow best when the plants they are infecting are growing quickly.

Some species of Pseudomonas are more active in the spring (P. syringae), while others may wait until summer to become fully active (P. cichorii). In late spring and early summer, expect to start seeing more Xanthomonas, especially on zinnia and ranunculus as they near the end of their production cycles. Their canopies close making an ideal environment for many pathogens to flourish.

Controls. Control of bacterial leaf spots in our trials has been best with copper products like Phyton 27 or Camelot alternated with the biological control product Rhapsody. It is not always possible to control bacterial diseases with chemicals so cultural methods including routine scouting, removal of diseased plants and keeping foliage as dry as possible are critical. Even when the most effective bactericides are used preventively, it is critical to use as many cultural controls as possible.

3. Pythium

Although there is certainly Pythium around the greenhouse, it may not be causing many symptoms since the weather is moderate and plants are under less stress than they will be later in summer. It is no time to relax however, because a mild Pythium infection in spring can result in major plant losses in summer.

Controls. Preventive treatment is needed for some diseases on a year-round basis and Pythium is one of them. Growers may have to deal with both cool-season and warm-season Pythium. Pythium species have different optimum temperatures, but I have not observed that products that work on one group act differently with the other group. Products for Pythium include phosphonates (like Aliette), etridiazole (Terrazole and Truban) and mefenoxam (Subdue MAXX). Be sure to alternate between at least two of these.

Resistance to mefenoxam has been reported with increasing frequency over the past 10 years. Only judicious use of all fungicides will keep them effective and available for ornamental use.

4. Warm-season downy mildew

While downy mildew on snapdragons, stock, alyssum and pansy may be less severe now, don’t relax yet. If you are growing salvias, digitalis, scabiosa and a host of other perennials, be on the look out for downy mildew.

What to watch for. Some downy mildew fungi are more severe in the cool winter while others actually are worse during spring when we mistakenly think it’s over for the season. Sometimes, cold damage mimics downy mildew symptoms and vice versa.

Since spring is a time of change, cold spells can still occur even though the weather is slowly warming up. Be sure you know what downy mildew symptoms look like. Send plant samples to a diagnostic lab if you are not absolutely sure of the cause.

Controls.  Downy mildew fungicides that perform the best in our trials include Stature DM, Aliette (and other phosphonates) and Heritage (or other strobilurins).

5. Powdery mildew

Spring and fall are ideal for both powdery mildew and rust diseases. The warm days and cool nights promote condensation on leaves (especially in unheated greenhouses), and rust-causing fungi and powdery mildew diseases rapidly take advantage of the situation. Many of these pathogens are already present in the plants and are just waiting for the optimum conditions to sporulate and appear on the surface.

What to watch for. Powdery mildew is common during winter on hydrangea, rosemary and gerbera. The pathogen is active in the spring as well. Be on the look out for rose powdery mildew, too. There are a wide range of symptoms for powdery mildew on miniature and greenhouse cut roses. Many cultivars appear to be almost immune to the disease, but you must thoroughly scout the entire crop.

Controls. Powdery mildew can be eradicated with a wide range of products from wetting agents (like Capsil and Latron) to biological controls (like Actinovate and Rhapsody) to traditional fungicides. The best products in our trials have been Compass O and Eagle.

6. Rust.

What to watch for.Rust pathogens on geranium, bellis, hypericum and aster will appear on the undersides of leaves. The pathogens are sometimes found as single pustules and at other times form rings of pustules. An early infection on a leaf may look like a simple chlorotic spot. Be sure to always turn the leaves over and examine the undersides.

Containerized gladiolus growers should be aware that rust has been reported in Florida. This is a quarantined pathogen and should be reported if found.

Controls. The most effective product for rust in our trials has been Heritage. Use a wetting agent if the rust has already formed pustules. The wetting agent does not control rust by itself as it can with powdery mildew. The wetting agent allows the fungicide to penetrate the rust pustules. Another very effective fungicide is Eagle (Systhane).

- Ann Chase

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Ann Chase is president, Chase Research Gardens Inc., 8031 Mount Aukum Road, Suite F, PO Box 529, Mount Aukum, CA 95656-0529; phone/fax (530) 620-1624; archase@chaseresearch.net; www.chaseresearchgardens.com.