Four Foliar Diseases That Will Drive Down Profits

They say not to judge a book by its cover, but in the ornamental industry, it is the other way around. People equate plant quality with appearance. And they rarely, if ever, shell out their hard-earned money for plants that aren’t in pristine condition.

Disease is one of the primary culprits in less-than-perfect-looking plants. These four pathogens, in particular, are worth avoiding in the greenhouse. Although not always fatal, these diseases will detract from crop aesthetics and make a dent in profits.

 

Powdery Mildew

Susceptible Crops

Many species of powdery mildew are host specific; others have a wide host range that includes poinsettias, roses, violas and zinnias.

Symptoms

Powdery mildew symptoms usually appear on the upper side of the leaves first, but they can also show up on the underside, especially on begonias and verbenas. A white, powdery growth, the result of thread-like hyphae strands and chains of spores (conidia), appears on infected plant parts. Young stems, shoots and foliage, as well as mature plant parts, are all prime targets for powdery mildew. Shoot distortion, leaf drop or stunted growth occurs, and necrotic lesions and chlorotic or purple spots may appear on leaves. Symptoms vary depending on the species of pathogen.

Environmental Conditions

Conditions during the spring and fall favor powdery mildew because of the potential for high relative humidity (>95 percent RH). High levels of relative humidity promote spore formation while low levels encourage spore dispersal. Different species of powdery mildew thrive at optimum temperatures. Air currents and worker activities also spread spores to other plants.

Management

Monitoring and sanitation, combined with chemicals, is the best way to control powdery mildew. The primary goal is to use chemicals preventatively to protect healthy plants. After eradicating existing infections, follow up with protective applications to prevent further outbreaks. Knowing the specific pathogen and host combination is helpful when making treatment decisions. Rotate chemicals with different modes of action to prevent resistance.

Adequate ventilation is important, as is proper plant spacing. Venting and heating at the end of the day helps reduce relative humidity during the night. Syringing or dousing infected plant material to wash off spores can be effective. However, if plants are prone to other leaf diseases this practice can be counterproductive.

 

Botrytis (Gray Mold)

Susceptible Crops

Wide host range, including gloxinia, gerbera, rose, snapdragon, geranium, poinsettia and petunia

Symptoms

Circular or wedge-shaped lesions appear on the leaves, eventually spreading to cover the entire surface. In severe cases, they grow down the petiole and onto the stem. Small, white, red or tan spots develop on flowers and begin to expand. Smoky gray, dusty spores appear on the plant. Damping-off occurs, as well as wilting, leaf spots and blight.

Environmental Conditions

Botrytis develops due to cool temperatures (below 85°F), high humidity and poor air circulation. Prolonged leaf dampness and dead plant material also contribute to this disease.

Management

Sanitation is extremely important for control of Botrytis, but the disease is so prolific chemical controls are also necessary for effective control. Several fungicides are labeled for Botrytis. Resistance can be a big problem. Rotate between chemicals with different modes of action. Botrytis cinerea populations have shown some resistance to the Benzimidazole class (thiophanate-methyl) and the Dicarboximide class (iprodione).

Promptly remove dead plant material and spent flowers and dispose of them in outdoor trashcans. Avoid injuring plants. Provide adequate ventilation and air circulation to keep humidity under control (< 85 percent RH). Vent warm, humid air to the outside and heat cool, incoming air to draw out relative humidity.

 

Impatiens Necrotic Spot Virus (INSV)/ Tomato Spotted Wilt Virus (TSWV)

Susceptible Crops

Wide host range. Roses and poinsettias do not seem to be host plants for INSV/TSWV.

Symptoms

Symptoms include spots, zonate spots, rings, mosaic and line patterns, necrosis near the leaf petiole, tip dieback, stem cankers and wilt. Patterns may be brown, black, yellow or white depending on the host plant. Stunting and yellowing are common.

Environmental Conditions

The probability of virus transmission increases when thrips populations are at their peak, generally in the spring and summer.

Management

Effective control of INSV and TSWV involves managing the vectoring insects that cause them, mainly Western flower thrips, but other species can vector as well. Isolate new plants on arrival until you can inspect for insects or presence of disease. Remove infected plants, and eliminate weeds in and around the greenhouse. Screen vents and doorways.

If thrips are not present, the disease can still spread through vegetative propagation of infected plant material. Avoid continuous cropping to break the disease cycle. Rotate insecticides to avoid problems with resistance, every two life cycles or approximately every three weeks.

 

Leaf Spots (Pseudomonas, Alternaria, Cercospora, Septoria, Myrothecium, Colletotrichum)

Susceptible Crops

Wide host range.

Symptoms

Symptoms vary with the host plant and disease organism, but usually begin as a black or brown spot (sometimes with a yellow halo) that begins to spread across the leaf and causes leaf drop/death. Flowers may also be affected. Stunting and poor plant vigor are also signs of trouble.

Environmental Conditions

Warm, moist conditions and extensive periods of wetness lead to problems with leaf spot diseases. Spores spread through splashing water. Depending on the species of the disease, it can enter the root system and spread through sub-irrigation systems.

Management

Minimizing splashing through the decrease or elimination of overhead irrigation can help control leaf spot diseases. In addition, make sure there is no free water on the leaves. Start with pathogen-free materials and rogue out infected plants. An accurate diagnosis of the disease-causing organism is important when making decisions about which treatment options will be the best fit.

 

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