A Newsletter for Professionals Growing Greenhouse Crops in the Rocky Mountain Region

Contents

• December is for Graduation
• Diseases of Cyclamen
• News from the Lab
• Time to Check Your Heat

December is for Graduation

The last semester of the millennium is fast coming to a close at Colorado State University. With it we have 33 fresh graduates leaving Colorado State University from our department. Of those, three will graduate from floriculture and are actively looking for employment.

In addition, we have two graduate students graduating with Masters of Science in Horticulture from the floriculture program. Each of them as completed an excellent thesis project.

Steven E. Newman, Ph.D.
Greenhouse Crops Extension Specialist
Horticulture and Landscape Architecture

Diseases of Cyclamen

Cyclamen have a reputation for being fairly finicky and a little more difficult to grow than other flowering potted plants. To add to this poor reputation, they are also susceptible to a number of diseases. Fungi, such as Botrytis can cause leaf spots and necrosis. Tomato spotted wilt and impatiens necrotic spot viruses can cause leaf distortion, leaf yellowing or graying, and even mosaic line patterns in leaves. However, two of the more common diseases encountered on this plant occur in the corm, roots and lower stems. These two problems are described below.

Fusarium wilt

This is a fungal disease that invades roots and eventually the vascular system of the plant, causing a vascular wilt. Roots may rot in young seedlings prior to the fungus invading the corm. Above ground, regardless of the age of the plant, symptoms begin as a yellowing of leaves. The leaves will characteristically yellow and wilt one after another, until the whole plant collapses. The most diagnostic characteristic is the purple, reddish brown, or nearly black discoloration of the vascular system of the corm. (To view this, just cut the corm open. Healthy tissue should be a creamy white color). The corm, also, will remain firm to the touch, a distinction, used to separate this disease from Erwinia soft rot.

Erwinia soft rot

Symptoms of this disease are similar to Fusarium wilt in that leaves will turn yellow, loose their luster, become grayish and wilted. This is a bacterial disease that invades fleshy tissues, such as corms. Once infected, the corms become mushy, rot and emit a foul odor. The bacterium may progress to the vascular system and result in stunting and yellowing of plants and collapse of stems.

Management

When generic symptoms of leaf yellowing, stunting and wilting are noticed, pull a plant out the pot and cut the corm. Presence of vascular discoloration indicates Fusarium. Rotted, mushy, foul smelling corms indicate Erwinia. Both causal organisms reside in the soil, can be moved in irrigation water, water splash, plant debris, soil, contaminated tools, and handling of infected plants. Regardless of which disease organism is identified, strict sanitation practices should be used. Destroy symptomatic plants, and clean up all debris on floors and benches. For Fusarium control: Avoid using growing media containing dark, decomposed sphagnum peat, as it is very conducive for the development of this fungus. Increasing the pH of the growing medium to greater than 6.0 will inhibit disease development. It appears that nitrogen fertilizer suppresses the disease, while ammonium nitrogen enhances it. Fungicides such as Chipco 26019, Cleary's 3336, Domain FL, and Terraclor are labeled for control, however chemical control is not entirely effective. Biological controls (bioantagonists) such as Deny, Mycostop, Root Shield can be used as a preventive tool. Once disease is present, however, these products are not as effective. For Erwinia control: Very few chemicals effective against this disease, sanitation is the sole management strategy.

Laura Pottoff
Extension Pathology Agent
Jefferson County

News from the Lab

Improving Postharvest Quality of Floriculture Crops

Postharvest quality of floriculture crops is a concern to every greenhouse grower, as well as the consumer. Our research objectives to improve postharvest quality include:

1. Determine the biochemical and physiological basis of flower senescence;
2. Use molecular biology to identify genes involved in the regulation of flower senescence and determine the role of the plant hormone ethylene in regulating these genes;
3. Utilize these senescence-related genes to create transgenic plants with longer-lived flowers and evaluate flower longevity and ethylene production in these plants; and
4. To investigate the feasibility of ethylene inhibitors for the commercial treatment of floriculture crops to increase flower longevity and quality and prevent petal abscission.

To achieve these goals, the Colorado Agriculture Experiment Station has established the research project, Improving Postharvest Quality of Floriculture Crops, to advance our understanding of these issues. Equipping the Postharvest Floriculture Laboratory is nearly complete and we now have some new faces who are actually doing much of the work. My first introduction is Dr. Felicity Johnson Potter.

Michelle L. Jones, Ph.D.
Assistant Professor of Floriculture
Horticulture and Landscape Architecture

Right to left: Dr. Felicity Johnson Potter, Dr. Michele Jones, and Jim Jones

I have recently arrived from Adelaide, South Australia where I completed my PhD in the Department of Botany. I am currently doing post-doctoral research in the laboratory of Dr. Michelle Jones at CSU, (Horticulture and Landscape Architecture). We are investigating the molecular basis of petal senescence in commercially important crops such as petunia and carnation.

A major project we are undertaking is the large scale screening of senescence-related genes. By taking a pool of expressed genes from senescing flowers and comparing this with a pool from fresh, non-senescing flowers, we can pull out those differentially expressed genes. Each gene, identified as senescence-specific, furthers our understanding of petal cell death. Whilst this may appear to be "basic research" at this stage, it has direct application to current floriculture practices, as well as laying the groundwork for generating the plant lines available in the future.

One of the petal, senescence-specific genes is the focus of another project, wherein we are currently generating new plant lines in our efforts to comprehend this particular, recently identified gene. We are investigating the role of this protein (the gene's product) that appears to increase during petal senescence (when many other plant processes are shutting down as the petal tissue dies). This protein falls into a group of proteins known as the "cysteine proteases". The current opinion on the cysteine proteases is that they breakdown general, cell proteins in the senescing petal cells, such that these subunits can be mobilized to the pollinated ovary, which is developing to produce seed. Here the subunits are reused to build more proteins. We aim to study the effects of artificially decreasing the amount of our identified cysteine protease, by making transgenic plants that have the cysteine protease gene "silenced". In this way we can find out its exact role in senescencing floral tissue and, in turn, how it affects the post-harvest product.

The model plant system used in Dr Jones' laboratory is Petunia hybrida. As well as being a plant that is readily genetically manipulated, it also represents a popular and highly marketed product.

Felicity Johnson Potter, Ph.D.
Postdoctoral Research Associate
Horticulture and Landscape Architecture

• Inspect the flue pipe. Wind is responsible for most damage to flue pipes, however, one can always expect some degradation of the joints in the greenhouse. Check for rusting unions and for any debris that may have collected in the flu pipe. If the flue pipe has an exhaust fan to move exhaust gasses through the flue, make sure that it is operating properly.
• Inspect the heat coils. Greenhouses are humid environments and metal equipment is subject to rust. If the heat coils are rusted through, exhaust gasses can contaminate the greenhouse environment.
• Inspect the gas manifold. Dirty gas orifices will cause incomplete combustion of the fuel, which will result in exhaust gasses that will contaminate the greenhouse environment.
• Check the ignition modules and gas valves. These devices do wear out and require periodic service. These devices should be inspected and serviced by a licensed technician.

Inefficient operation of gas fired unit heaters can lead to a lot of problems in the greenhouse. Primarily we think of carbon monoxide, which is deadly to the staff, but one must also think about ethylene gas as well.

Ethylene levels as low as 20 ppb (that is parts per billion) have been shown to damage Cattleya species and 500 ppb are sufficient to cause flower abortion in tomatoes. Concentrations of 50 ppb for extended periods (how long? two to four hours) are just as deleterious as high concentrations.

There has been some interest in using CO detectors for estimating ethylene in a greenhouse. In Holland they use these detectors. They conclude that the ethylene level would be less than 0.1 of the critical 50 ppb if the CO content of the undiluted flu gasses did not exceed 50 ppm. The presence of CO, however, does not guarantee the presence of ethylene and vice versa. But they are cheap. Some growers use tomato plants underneath their unit heaters and if the leaves exhibit epinasty, they assume that there is ethylene contamination. Tomato plants are typically more sensitive than other floriculture crops to ethylene. (you can read more in Dr. J.J. Hanan's text, Greenhouses: Advanced Technology for Protected Horticulture)

Prevention is the key to ethylene gas control in the greenhouse. Maintain your gas-fired heaters in good condition. Clean the manifolds regularly and check for cracks in the heat exchangers. Flue pipes must be clean and free of debris. They must also have the correct clearance over the building if they are not connected to a forced air exhaust system. IR-radiant heat systems are not immune to ethylene contamination. Mount the exhaust fans as close to the end of the flue as possible to prevent any back pressure from a prevailing wind. Finally, make sure that your gas supply is adequate for the unit heater and that you are supplying adequate oxygen for combustion.

If you suspect that you have an ethylene gas problem, contact Michelle Jones and we can have your greenhouse atmosphere sampled for a small fee.

Steven E. Newman
Greenhouse Crops Extension Specialist
Horticulture and Landscape Architecture

Upcoming Events

December 7, 1999

1999 Poinsettia Open House
W.D. Holley Plant Environment Research Center
630 West Lake Street
Fort Collins, Colorado

December 18, 1999

January 10-14, 2000

January 19-22, 2000

Wyoming Groundskeepers and Grower's Association
Casper, Wyoming

January 28, 2000

February 5-14, 2000

Colorado Garden and Home Show
Denver, Colorado