Take aim to battle 3 common diseases

Here’s a rundown on three threatening diseases and how to combat them.

Botrytis blight

Botrytis is a familiar disease problem. It causes stem, leaf and flower blights. Botrytis is well-known for its ability to produce large masses of gray spores (hence the name “gray mold”) that may be picked up, carried on air currents and transported to healthy plants.

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On bedding and stock plants, Botrytis typically becomes established and produces spores on older lower leaves near moist growing medium and under the plant canopy. Botrytis also readily infects the broken or cut stem surface of stock plants and progresses downward, causing a dieback of the entire stem. Abundant spores can be produced on any diseased tissue.

While fungicides alone cannot control Botrytis, they are necessary components in a successful disease-management program that includes environmental control and sanitation. Even the best fungicides will fail if the relative humidity exceeds 85 percent and the leaves stay wet for six hours or more.

Plants may become wet from water dripping from overhead, dew or condensation. This moisture allows Botrytis spores to germinate and penetrate the plants.

Watering in the morning so that the foliage can dry rapidly is one way to minimize Botrytis. Practices that reduce the relative humidity are also helpful and include spacing plants farther apart and providing good air circulation.

Treatments for Botrytis

There are several fungicides to consider for Botrytis. Use Daconil, Decree, Dithane, Protect T/O, Mancozeb and Endorse as protectants. Use Chipco 26GT, Heritage, Compass and Insignia as systemics.

They can also be divided into two teams. Use Daconil (chlorothalonil), Decree (fenhexamid), Chipco 26019 or Chipco 26GT (iprodione) or Endorse (polyoxin D zinc salt) as the A team for the first line of defense. The B team is comprised of Compass (trifloxystrobin), Heritage (azoxystrobin), Insignia (pyraclostrobin) and mancozeb products (Dithane, Mancozeb and Protect T/O).

Powdery mildew

Powdery mildew epidemics can occur without warning. Depending on how the disease is managed, there may be minimal impact or complete crop loss. The white talcumlike colonies are initially small but can rapidly blight leaves, stems and flowers of susceptible plants.

Growing crops susceptible to powdery mildew can be a challenge, and fungicides have typically played a key role. Powdery mildews are tricky and have been known to genetically adapt to overcome some of the most effective fungicides. Since the powdery mildews are a problem each year, a long-term strategy is needed.

Treatments for powdery mildew

For powdery mildew, the fungicide A team could be Eagle (myclobutanil) and Terraguard 50W (triflumizole). The second line of defense has a few more options: Heritage (azoxystrobin), Compass (trifloxystrobin), Insignia (pyraclostrobin), Zyban (thiophanate-methyl plus mancozeb) and Strike (triadimefon).

Downy mildew

Certain crops such as roses and snapdragons have a long history of downy mildew. Others, such as coleus and impatiens, have only recently had problems with this disease.

Downy mildew is nasty. It can cause a lot of damage in a short period and is tough to control. Downy mildew distorts plants and blights foliage. Severely infected plants often drop their leaves.

The undersides of blighted leaves become fuzzy when laden with downy mildew spores. The spores can be carried from diseased plants to healthy plants by air currents. Since the spores are small and microscopic, they easily escape detection. Yet, these spores can be hitchhikers on plant surfaces. If moist conditions exist, downy mildew spores can travel long distances on plant surfaces.

If a thin film of water forms on the plant’s surface, the spore will germinate and penetrate the plant. Once downy mildew is inside the plant, the pathogen forms a network of fungal threads that serve to withdraw nutrients. If conditions are moist and cool, downy mildew may continue to grow and develop within the plant causing spotting or blighting of the foliage.

Once downy mildew is well-established, the fungus begins to reproduce, making more spores. These spores, which can be seen on the underside of the plant’s leaves, appear fuzzy or velvety because so many are clumped together. Spore clumps are easiest to see in the morning or during moist conditions. A 10X hand lens can help.

A different scenario can play out when downy mildew has successfully penetrated a plant but the environment becomes hot and dry -- conditions that do not favor the pathogen. The downy mildew becomes quiet, neither growing further within the plant nor dying out. Plants with “quiet” downy mildew infections may appear healthy. Plant pathologists don’t fully understand this phase of disease development. It isn’t known what triggers a “quiet” infection to become active or how long the quiet phase can last.

The downy mildew quiet phase causes confusion for growers because a plant that appears healthy may develop disease almost overnight. Downy mildew can be carried on the seed in some crops. Since downy mildew can ramp up quickly, preventive measures are always best.

Treatments for downy mildew

For downy mildew, it’s important to alternate or mix protectants with systemics. Downy mildew is a pro at mutating, so applications should be thorough and timed correctly. It’s best to apply fungicides frequently (every five to seven days), depending on the weather. Good plant coverage is key, even with systemics.

Systemics for use on downy mildew include: Heritage (azoxystrobin), Compass (trifloxystrobin), Insignia (pyraclostrobin), Subdue MAXX (mefenoxam) as a drench, Aliette (fosetyl-al) and FenStop (fenamidone). Protectants include mancozeb, copper and Stature DM (dimethomorph). NOA 446510 (mandipropamid) and V-10161 (fluopicolide) are promising tools that are in the works.

Here are two sample programs to battle downy mildew.

For an intensive program, apply a Subdue MAXX drench at planting. Then apply foliar sprays every seven days with this regimen:

* Heritage + mancozeb.

* Stature + mancozeb.

* Aliette + mancozeb.

* FenStop + mancozeb.

For a protectant program, again apply a Subdue MAXX drench at planting. Then apply foliar sprays every seven days with this regimen:

* Mancozeb.

* Heritage + mancozeb.

* Stature + mancozeb.

* Mancozeb.

* Aliette + mancozeb.

* Mancozeb.

* FenStop + mancozeb.

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This research was funded by Cooperative Agreement #59-1907-5-553 with the USDA-Agricultural Research Service and the American Floral Endowment.

- Mary Hausbeck

April 2008