Greenhouse Disease Management
Rusts
One serious disease that can negatively affect production of many ornamental crops is a fungal infection called rust. The common name “rust” refers to the characteristic reddish to orange blister-like swellings (known as “pustules”) produced on the infected leaves and stems of host plants. Host plants are rarely killed, but their overall plant health and appearance is adversely impacted. There are many different species of rust, affecting many different greenhouse plants. Most species of rust infect a single plant family, although some regularly swap hosts during their life cycle. Rust diseases can affect a wide range of ornamental host plants including aster, campanula, carnation, chrysanthemum, daylily, fuchsia, goldenrod, hollyhock, iris, marigold, penstemon, poinsettia, rose, snapdragon, switchgrass, statice, veronica, violet, and zonal or bedding geranium to name a few.
Disease Cycle
Rust diseases are caused by a group of highly specialized fungi with complex life cycles. Unlike most fungi that produce only one or two types of spores on a single host, rust fungi have a more complicated life cycle. For some rusts, only one host is needed. For others, two different types of hosts are needed for the pathogen to complete its life cycle.
Symptoms
Infected plants develop lesions (pustules) on the lower surfaces of plant leaves which increase in size and eventually rupture the epidermis and release spores (See Figure 28.10). These spores are typically brightly colored and are characteristic in diagnosing rust infections. Pustules also can be present on the upper surface of leaves and can coalesce to form large necrotic areas.
Cultural Management Strategies
Rust diseases can be spread through the air via wind-blown spores, by splashing water within a greenhouse, or through the introduction of infected plants in the greenhouse. Rust spores can also spread plant to plant through splashing water. Rust fungi can produce large quantities of spores in a relatively short period of time, so even a small number of infected plants can cause an epidemic. Cool temperature, high canopy relative humidity, and plant density are critical to initial disease development and for secondary and tertiary infection that can typically occur. Carefully inspect all incoming plants for symptoms, especially those that occur at the beginning of the production cycle.
Chemical Management Strategies
There are several effective fungicides for controlling rust diseases, but these are preventative, not curative. Fungicide treatment is no substitute for sanitation. Rotate applications between chemical classes or FRAC codes to help prevent fungicide resistance from developing. Use contact fungicides in rotation with systemic fungicides. The best rust fungicides usually fall into the strobilurin or sterol inhibitor groups.
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