Crystal Ball Project
GIE Media is hosting the Horticultural Industry Crystal Ball Project June 5-7 in St. Louis. We’ve invited some of the top greenhouse and nursery producers to discuss critical issues such as energy costs and the use of alternative energy; labor; water uses, shortages and conservation; technology advances; distribution patterns; and communications, including social media. Watch for stories and videos from this event in upcoming issues and on our website.

 
Correction
The gerbera pictured on Page 66 of the New Pot Plant Varieties in May Greenhouse Management is Floriline Midi Monet.

JJohn W. Bartok Jr.

Q: Sub-irrigation: a good way to provide containment

There are several areas of concern covered under runoff regulations, including water from greenhouse roofs, runoff from driveways and parking areas, excess interior irrigation water and leaching from exterior growing areas. Subirrigation is one method that contains irrigation water. Instead of letting leachate drip from benches or ground beds into the soil, the water is collected, stored and recirculated.


Containment advantages
Besides the benefit of reducing runoff, there are several other advantages:

• Less labor. Most of the time needed for the setup and operation of an irrigation system is eliminated.
• Uniform plant growth. Every plant gets the amount of water it needs.
• Less water is needed. A savings of 50 percent or more of water can be realized.
• Less fertilizer is needed. Without leaching, fertilizer rates can be reduced by 25–50 percent.
• Lower humidity. Because the foliage remains dry, there is less evaporation.
• Less disease. As there is little water movement between containers, spread of disease is limited.
• Increased space efficiency. With the use of bench and floor systems containers can be spaced more efficiently.

Containment system concerns
Challenges to ebb-and-flood irrigation include:

• Greater system cost. Depending on the system installed, cost can be $10-$20 per square foot of growing area.
• Learning curve. Production techniques are different than conventional production with overhead irrigation.
• Greater monitoring of irrigation solution. Electrical conductivity and pH have to be monitored on a regular basis.

System components
In addition to troughs, benches or flood floors, the basic containment system consists of tanks to collect and hold the water or nutrient solution, pumps and piping to move the water and a control system that regulates when the system is to operate and for how long.

Concrete or polyethylene tanks are sized at ½- to 1-gallon capacity per square foot of growing area. Two or more tanks are usually installed for adequate water and nutrient storage. PVC pipe and fittings give good service for handling the water or solution.

Stainless steel centrifugal pumps are used to handle the nutrient solution and filters screen out materials such as medium debris and leaves. Timers or a controller are needed to adjust the length of time the pumps operate.


Small budget solution
Trough system. This system allows the use of existing benches reducing cost. PVC or aluminum troughs, wide enough for the pots to set in are placed on the benches. Benches are adjusted to provide a slight slope so the water drains from the supply to the discharge end.

Water is pumped from the reservoir tank under the benches to the high end of the troughs. A cross gutter at the discharge end collects the water and piping carries it back to a holding tank. Water from the holding tank is pumped to the reservoir tank, thus the closed system. Trough systems can also be suspended from overhead trusses to gain additional growing space.

Ebb-and-flood on existing benches. In this system, the benches remain level and plastic liners are inserted to contain the water. Water from a reservoir is pumped through a two-way valve to the bench. When the level of water reaches ½ to 1 inch and the growing medium in the containers has absorbed as much as needed, the valve reverses and the remaining water is siphoned out.

Research at Geremia Greenhouse in Wallingford, Conn., has shown that partial saturation by controlling the time that water is left on the bench can restrict uptake of water resulting in higher quality plants. It can also affect the rate of plant growth reducing the need for growth regulator applications. Partial saturation is accomplished by adjusting the time that the supply pump operates using an environmental controller.

Every disease has at least one season that favors its development. For Rhizoctonia it’s the summer. This fungus usually prefers the heat, humidity and rainfall that occur in many parts of the United States during the summer. In the southeastern U.S. it can also be found in spring and fall when the environment is still favorable for its development.


1. What is the plant host range of Rhizoctonia?
There are several types of Rhizoctonia that attack ornamentals and they have very little host specialization, meaning one can attack many types of plants. Rhizoctonia solani is the most commonly found and tends to have a very broad host range. In contrast, binucleate (two nuclei per cell) pathogens are most common on woody ornamentals with aerial blight in the southeastern U.S. Still other types of Rhizoctonia attack turf grass but they are not often found on ornamentals. The same R. solani isolate that can cause leaf spot on aglaonema can cause cutting rot on poinsettia and damping-off on celosia.

Rhizoctonia root rot: Aglaonema, azalea, begonia, campanula, ferns, holly, impatiens, juniper, philodendron, poinsettia and spathiphyllum.

Rhizoctonia stem rot: Aster, dianthus, impatiens, poinsettia and pothos.

Rhizoctonia aerial blight (also called web-blight): Azalea, holly, juniper and pittosporum.

Rhizoctonia bulb rot: Calla lily, iris and gladiolus.

Rhizoctonia cutting rot: Azalea, hydrangea, osteospermum, pittosporum, poinsettia, rosemary and Vinca minor.

Rhizoctonia damping-off: Celosia, gomphrena, vinca, impatiens, stock and snapdragon.


2. What conditions favor Rhizoctonia?
Rhizoctonia aerial blight and leaf spot occur primarily in the warmer months when high temperatures coincide with high moisture levels. This is common in the southeastern U.S. when rainfall and temperatures both peak. In other areas of the country, the nurseryman creates ideal conditions for these diseases by growing plants in covered structures which are heated and irrigated overhead.


3. What are some of the symptoms of Rhizoctonia?
Rhizoctonia spp. usually attack plants at the soil line, causing constriction of the stem which results in girdling and death of the plant tops. The pathogen can attack leaves as well and is especially severe when plants are grown close together and leaves are wet.

Entire stock beds or flats can be lost to Rhizoctonia aerial blight in very short periods of time, sometimes in a matter of a few days. The pathogen is soil-borne which means it lives in the soil or growing medium and causes both pre- and post-emergence damping-off of many seed produced crops.

Disease development can occur in a couple of days so plants should be scouted and monitored for symptoms at least weekly. Brown, irregularly-shaped spots form anywhere on foliage or stems. The spots can be up to 1 inch across and sometimes have concentric rings (target-like).

When humidity is high the web-like brown mycelium of the pathogen can cover portions of the plant which are infected. This can be seen many times in aerial blight and sometimes when flats of seedling crops are attacked.

Rhizoctonia stem rot is typified by brown cankers forming at the soil line. The cankers are sunken and dry appearing and result in collapse of the plant tops. The canker can be very constricted and is sometimes called wire-stem on field-grown crops like beans and cotton. It is also sometimes only found on one side of the stem and is called sore-shin (especially on cotton).

One of the most typical expressions of Rhizoctonia on ornamentals is cutting rot. This can occur within a week of sticking cuttings and looks like many cuttings rots. The most typical cause of cutting rot has been Rhizoctonia in my experience.

Root rot caused by Rhizoctonia appears much like root rots caused by other fungi. Roots become brown, somewhat mushy and disintegrate. An accurate laboratory diagnosis is therefore crucial since fungicides that control Pythium do not control Rhizoctonia and vice versa. Keep in mind that mixed infections, cases where two pathogens are active, are also common.
 

Quick Control Tips:
Only an accurate diagnosis allows you to choose the best and most complete control strategies. Be sure to watch plants for above ground signs of root rot, including yellow or pale green leaves, small leaves, wilting and stunting.

Soil-borne pathogens such as Rhizoctonia spp. can escape notice and therefore cause serious losses before they are identified. Prevention is therefore the key to controlling Rhizoctonia diseases.

Use healthy plants, new or sterilized pots and growing media and avoid placing plants directly on the ground. Try to increase space between plants if possible and manage overhead mist to minimize the amount of water or time with active mist in a propagation area. If you follow these steps the fungicides you apply will have the greatest chance to work. As mentioned earlier, Rhizoctonia spp. occur in the potting medium and can cause stem and leaf diseases, making the application site for a fungicide critical.
 
The most effective fungicide for Rhizoctonia diseases in trials at Chase Horticultural Research has been fludioxinil (Medallion 50W). This active ingredient is also found in Palladium (a combination with cyprodinil) and Hurricane (a combination with mefenoxam).

Extensive testing by many universities on many ornamentals has shown that Medallion is extremely effective for Rhizoctonia diseases. Medallion is not safe for use as a drench on impatiens and New Guinea impatiens. Most other plants tested were not adversely affected by Medallion.

Two other very effective fungicide groups include strobilurins (MOA 11) and benzimidazoles (MOA 1).

For biological control, our trials have shown that RootShield (Trichoderma harzianum) can be very effective. Keep in mind that it is most effective when used on Rhizoctonia root rot and not leaf spot or aerial blight and that it must be used preventively.

Have a question? You can write Ann at archase@chaseresearch.net.

Pesticide resistance can occur with repetitive use of the same chemicals. A resistance management plan is critical to avoiding pesticide resistance and to sustaining the use of limited available products. Here are some tips to consider when developing a resistance management plan.

Quick Tip 1
Minimize pesticide use. Reduce pesticide use by scouting crops regularly and applying pesticides only when pests are present at threshold levels, targeting the most susceptible life stages. Incorporate non-chemical strategies such as installing exclusion screening, altering the environment to control foliar pathogens and incorporating biological controls.


Quick Tip 2
Know chemical mode of action. Resistance management strategies include rotating pesticides with the same mode of action or group number. It is therefore important to know the mode of action for the active ingredient of each pesticide that you use. The mode of action can be found on the pesticide label and on the Useful Websites listed at the end of this article.

The mode of action is the mechanism by which the pesticide kills the pest and is different than the chemical class. Rotation schedules should not be based only on the chemical class because these pesticides may have the same mode of action. For example, organophosphates and carbamates, despite being different chemical classes have identical modes of action (interfering with the nervous system of the target insect).


Quick Tip 3
Look for pesticide properties that do not promote resistance. Avoid the use of persistent chemicals. An ideal pesticide quickly dissipates so that pesticide residuals do not degrade over time to a concentration that will kill only the susceptible individuals within a population. When persistent chemicals must be used, consider where they can be used in a rotation scheme to provide the control needed and with a minimum length of exposure.

Products with multiple modes of activity can be used anytime in a rotation scheme. These include pesticides that kill by desiccation or smothering (insecticidal soap, horticultural oils), selective feeding inhibitors and biopesticides.

In contrast, conventional insecticides and miticides tend to have site-specific modes of activity. These pesticides inhibit one vital function or target site. Also, these pesticides must be managed carefully because it only takes a single-gene mutation to develop resistance.

Pesticides can be selective or nonselective (broad-spectrum). Selective pesticides kill only certain groups or species of plants or animals and are generally preferred in IPM programs. Use of selective pesticides in combination with biological control fosters reduced pesticide use and avoids the buildup of pesticide resistance among pest survivors through parasitism or predation.

Check fungicide labels and group information for the risk of resistance development. Know which fungicides are rated as high risk and follow label instructions for managing fungicide resistance. Most of the newer fungicides have a single-site mode of action which favors the development of resistance.

Look for fungicides that are multi-site inhibitors (mode-of-action group M), denoting a low risk of resistance development. Include these multi-site inhibitors in your rotation schedule.
 

Have a question? You can write Julie at jpnewman@ucdavis.edu.