6 diseases to watch for in fall

As fall approaches, there are several diseases that you should be treating preventatively. As with other seasons, some pathogens prefer the conditions of fall while others are a problem because of crop timing.

1. Black root rot on pansy

Black root rot, caused by Thielaviopsis basicola, is most troublesome on pansy but also occurs on salvia, petunia and vinca. A few years ago, there was a serious outbreak of black root rot on poinsettias. Check the roots on these plants and send samples of unhealthy ones to a diagnostic lab for identification if you are unsure of the cause.

It has long been held that keeping the growing medium pH below 8 and above 4.5 minimizes T. basicola. For pansy production without the disease, a pH of 5.5-6 seems to be best. Production of cool-season crops like pansy in the late summer is a recipe for plant stress and can lead to outbreaks of black root rot.

Reusing plug trays creates the potential for recontamination with the disease a serious concern.

Controls. Research has shown that cleaning trays and benches with 10 percent sodium hypochlorite (bleach) in combination with a detergent and scrubbing is best. ZeroTol was effective when sprayed onto plug trays at 2.5 ounces per gallon. GreenShield was effective when used as a 10-minute dip at 1 tablespoon per gallon.

Fungicide control trials continue to be reported every few years with mixed results for fludioxinil (Medallion), azoxystrobin (Heritage) and triflumizole (Terraguard). The only product that works consistently between researchers and trials is thiophanate methyl (3336) used on a 14-day interval as a 16 ounce per 100 gallon drench.

A comparison of thiophanate methyl fungicides at the low and high label rates showed similar results for 3336, Fungo Flo, Fungo WSB and Banrot. The lower rate of each fungicide failed to give adequate control while the higher rates tested provided the best control.

2 and 3. Powdery mildew and rust

I wrote about spring diseases in GMPRO March (Page 45) and now that it is fall it’s once again time for powdery mildew and rust.

Days are still warm but nights are starting to cool down. Condensation may occur overnight and both rust and powdery mildew fungi are ready to take off. While it is often possible to stop an outbreak of powdery mildew, it is critical to prevent rust diseases.

Watch roses, gerbera, hydrangea, verbena, salvia, scabiosa and other bedding plants and perennials for powdery mildew. Rust diseases, especially those on woody ornamentals, can be prevalent. Some fungi have been living on the host all summer just waiting for favorable environmental conditions.

Controls: Since last spring we have conducted additional trials applying wetting agents alone for powdery mildew or in combination with a fungicide for rust. We found that Capsil provided some control of powdery mildew on gerbera when used at 4 ounces per 100 gallons, but failed to give any control at 1 ounce per 100 gallons.

On hypericum rust, use of Sync (16 ounces per 100 gallons) with Heritage (4 ounces per 100 gallons) gave excellent results. Use of Heritage alone was ineffective.

Sterol inhibitors (Eagle, Strike and Terraguard) can be used. Additional sterol products (Trinity from BASF) are being developed for both powdery mildew and rust. Strobilurins (Compass O, Heritage and Insignia) are very good controls for rust and powdery mildew and should be rotated with a sterol inhibitor for best results.

4. Pythium root rot

Poinsettias are part of many growers’ fall production and are scheduled for a narrow marketing window. The most common disease of these plants in fall is Pythium root rot. If poinsettia cuttings make it through rooting without a Rhizoctonia stem rot or Erwinia blight outbreak, they can develop Pythium root rot later in their lifecycle.

Pythium symptoms include wilt even when the growing medium is wet, slow growth, poor color (especially yellow lower leaves), weakened stems and, of course, root rot. Roots appear gray or water-soaked and disintegrate easily when handled. They often show the classic Pythium characteristics of retaining their central core while losing the outer cortex of the roots.

Controls: Due to the critical timing of poinsettias it usually pays to apply fungicides preventatively. Etridiazole (Banrot, Terrazole or Truban) is used in some portions of the country while in others Subdue MAXX or Aliette is the prevention of choice. In still others, RootShield or PlantShield (Trichoderma harzianum) is used exclusively with excellent results. Poinsettia is one of the most responsive crops to the biological protection afforded by Trichoderma.

Rotate between fungicides to prevent resistance. Experience with etridiazole for the past 30-plus years has shown no resistance in Pythium populations. In contrast, some Pythium populations have become resistant to mefenoxam (Subdue MAXX). If you have relied on Subdue MAXX alone and are not happy with results, you may have a resistant strain of Pythium.

Phosphonates (phos acid alternatives), including Aliette, show varying degrees of success for Pythium on many plants.

5. Botrytis stem rot.

One of the last diseases to consider in poinsettia production is late-season Botrytis stem rot. Losses due to Rhizoctonia usually occur during the first month of poinsettia production. At this point in production, the lower temperatures make Rhizoctonia stem rot unlikely and Botrytis stem rot common.

Although growers routinely apply fungicides for Botrytis blight on bracts and petioles they do not realize that some cultivars are very sensitive to stem rot caused by the same fungus, Botrytis cinerea.

Controls. It can be very hard, if not impossible, to apply a Botrytis fungicide to a crop that has reached final spacing and the flower canopy has closed in. I usually recommend a final spray of Decree (16 ounces per 100 gallons), Medallion (4 ounces per 100 gallons) or Chipco 26019 (16 ounces per 100 gallons) directed to the stem base before the canopy closes.

6. Cyclamen anthracnose

Late last summer and throughout fall an outbreak of Cyclamen anthracnose, Glomerella (Colletotrichum) appeared in Florida, Texas and California. Although the disease occurs in Europe rather routinely, it has not been widespread in the United States for the past 30 years.

Spots appear on leaves and rapidly enlarge up to 1/2 inch wide. The spots are sunken, brown to black and sometimes coalesce to form large, blighted areas. In one of our trials, the flowers were also infected and resulted in their complete collapse.

Controls. We did some chemical control trials over the winter and found a variety of products that are very effective in preventing this disease. Excellent control was seen with Clevis (2 pounds per 100 gallon) and Pentathlon (24 ounces per 100 gallons). Rhapsody (1 percent), Daconil Ultrex (1.4 pounds per 100 gallons and Chipco 26019 (1 pound per 100 gallons) also gave a high degree of control over flower anthracnose. No significant control was seen with Terraguard (8 ounces per 100 gallons).

In another trial, optimal control was seen with Daconil Ultrex (1.4 pounds per 100 gallons. However, flower damage and residue were extreme.

Heritage was also excellent in preventing anthracnose on cyclamen at 2 or 4 ounces per 100 gallons with or without Capsil.

Palladium (a combination of fludioxinil and cyprodinil) is under development. In this trial, it gave very good to excellent control as the rate was increased from 2 to 6 ounces per 100 gallons.

Medallion alone at 4 ounces per 100 gallons gave very good control but it was not as effective as Heritage or Palladium in this trial.

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- Ann Chase

Ann Chase is president, Chase Horticultural Research Inc., (530) 620-1624; archase@chaseresearch.net; www.chaseresearchgardens.com.