Collect plant samples

The first step in solving insect and disease problems is to correctly identify the cause. This often involves sending plant samples to a diagnostic clinic for verification of the agent causing damage. To help the diagnostician correctly identify the pathogen, you must know how to collect samples and properly submit them. You must also provide the correct information to ensure that the problem will be diagnosed quickly and correctly.

Landscape plant samples fall into four categories: herbaceous annuals and perennials, turf, shrubs and trees. Although the specific sampling requirements for each type of plant may differ, you must follow five steps to properly take a plant sample. If you follow these steps and integrate them with the top 10 tips for sample collection (see sidebar on this page), you will be prepared to face insect and disease challenges.

Five steps for collecting samples

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  1. Map it out. Record the location of the infected plants. Also, note their proximity to healthy ones. Look around the site for other factors that may influence disease or insects. Irrigation, shading, and proximity to sidewalks, parking lots and buildings are some environmental factors that you should look for. When mapping trees, look for damage such as construction that may have occurred in the past.

    The map will help you determine if the disease is spreading, and it will help the diagnostician form a picture of what the site is like.

  2. Dig up the plant. Include both roots and soil with your sample. Symptoms you see on a plant may be the result of root infections or poor soil conditions. Diseases often form complexes of several pathogens that attack the plant once it is stressed. However, you need to know the reason for the initial decline. For the diagnostician to fully assess the plant, you must include as much plant material as possible.

  3. Package the sample. When packaging a sample, keep the different parts of the plants separated. Wrap the roots and tie the plastic around the stem just above the soil. Next, cover the top of the plant. Loosely tie the plastic so that air exchange occurs. You do not want the sample to rot. Try to ship your samples in boxes so that they are not damaged. Fill empty spaces in the box with wadded newspaper. This will keep the sample from becoming damaged during shipment.

  4. Provide information. Most diagnostic labs will have a sample submission form that you can download from their website. Fill it out completely. Provide a copy of the map and include any other information that may affect the diagnosis.

  5. Ship it quickly. Many diseases and insects spread rapidly, so you may have to ship your samples with two-day or overnight delivery. The sooner the samples reach the lab, the more accurate the diagnosis will be.

The following discussion includes real-life situations that you may eventually encounter. It will also give you a feel for what happens at a diagnostic lab. The discussion is broken down into the four types of landscape plants that you may need to sample. Where necessary, I have included the differences in sampling procedures for that particular plant type.

Bedding plants

Responding to a terse call about landscape problems, you arrive at the corporate headquarters of your largest client. First, you notice trouble with the circular bed, 20-foot diameter, near the main entrance. It is planted with marigolds, creeping thyme and Madagascar periwinkle to form a resemblance of the company's logo. A few feet from one edge, several periwinkles are dead, and a few scattered ones are showing dead branches. However, the marigolds and thyme are fine. You don't see any insects, and you know the bed is uniformly irrigated and fertilized. How would you take samples to confirm and identify a disease problem?

  • Digging bedding plants. Because bedding plants are small, carefully dig three to five of the affected plants. Choose ones that show a range of decline from early stages to near death.

  • Wrapping bedding plants. Place each plant in a separate plastic bag. Gently wrap newspaper around the foliage.

A seasoned diagnostician will likely isolate for the fungus Phytophthora from the dying stems of the periwinkle and e-mail you a response within a few days. If the lab can diagnose the problem by symptoms alone, you may receive notification the next day.

Turf

The manicured sweep of fescue lawn bordering the parking spot reserved for the CEO has several dead patches ranging in size from 3 to 5 inches in diameter. The problem appears to be in the early stages, but you're unsure whether this is brown patch or pythium blight. Carefully dig samples from two of the areas making sure the root system is intact.

  • Digging turf. Dig a 6-inch diameter by 4-inch deep plug from the leading edge of the dying turf patches. Your sample should contain both affected and non-affected grass. The diagnostician will look for active fungi in that area.

  • Wrapping turf. Wrap each plug in wax paper or several layers of newspaper to help hold the soil intact. Place the plug in a plastic bag for shipping. Make sure the plugs are packed tightly with crumpled newspaper to prevent shifting and loosening of the soil during shipment.

The diagnostician performs a quick check of leaf lesions. They display the typical strands of Rhizoctonia, which confirm brown patch.

Shrubs

Finally, your attention is called to the skip laurel (Prunus laurocerasus) hedge near the entry. One of the shrubs has a couple of dying branches. The leaves on another shrub have changed from a dark, lustrous green to an ashy-gray green and it also has several dead branches. On the main trunk, 6 inches above the soil line, you note the bark has some sunken areas and a couple of small holes. On several other plants, you also notice irregular holes in the leaves that appear to be eaten away by a caterpillar. However, your thorough search of the hedge shows no insects on the leaves. You determine that the irrigation system is working properly and that no other obvious cause of damage exists.

These shrubs are 3 feet tall with a root ball that weighs 50 pounds, and the gray-looking plant probably will not recover. You decide to sacrifice it to the diagnostic lab.

  • Digging shrubs. Use a spade or shovel to dig around the root ball. Don't pull the plant up; you will tear weakened roots. Obviously, the cost of shipping a large shrub is prohibitive. With most landscape shrubs the entire root ball is not needed. Cut it in half. However, retain for shipment the side that displays symptoms. Reduce the size further by removing some additional soil. However, make sure that you retain many of the small, fibrous roots. These are needed to check for some root rots. The major branches all look identical so remove a couple of these about halfway down the plant. Trim the remaining large branches back, but keep some with the sample.

    If the infected shrub is too large to dig, remove some of the branches that are affected. Also, be sure to dig up some of the small feeder roots and 1 quart of soil. Place them in the same plastic bag.

  • Wrapping shrubs. Use a large garbage bag to contain the root system and soil. Cover the top of the plant for shipping. Also, take a sample of roots and soil from the adjacent plant that is showing symptoms. Take a third sample that consists of the leaves with holes. Label each bag accordingly.

The diagnosticians will check soil pH and total soluble salts in the soil to rule out cultural problems. If the roots are decayed, the lab will attempt to isolate the pathogen. They will look at the entire main stem on the whole-plant sample to check for mushroom root rot. By peeling some bark at the soil line, they will be able to look for the characteristic white, fungal mycelium. They will also note the holes in the main stem and call on an entomologist to check for borers. Next, they will check the branches for signs of fungal diebacks. Then they will investigate the leaf spots and irregular holes in the leaves.

An interim diagnosis report arrives by e-mail the next day and indicates that the entomologist has found peach tree borer in the main trunk. The root-rot test results will be ready in 3 days. The leaf spot is identified as Xanthomonas bacterial leaf spot, otherwise known as shothole. It is a problem where overhead irrigation is used. Three days later, the final report comes in confirming Phytophthora root rot on both root samples. The borer was attracted to the weakened plant and hastened its decline.

Trees

A large oak tree at the edge of the property that was not cleared for the building has developed significant dieback. Also, you notice a large yellow mushroom-like structure in the leaf litter about 3 feet from the base of the tree. Upon closer examination you see that it is arising from one of the major roots of the tree.

  • Sampling trees. Trees are a challenge because you cannot send them in for shipment. You must look for symptoms or clues that can be sampled. In this case, you remove the mushroom and prepare it for shipment to the lab. You should also sample the branches by cutting them several inches beyond each side of the affected area. Include live wood in the sample. Also, collect some fine roots and soil.

It may take a little longer to sample correctly, but getting a rapid and accurate answer to your landscape problems is worth the investment. By identifying the pathogens in your landscape, you will avoid misapplications of controls. While many fungicides offer broad-spectrum control, they cannot control everything. Also, some turfgrass controls may be labeled for use on ornamentals while some are not. Once you know your pathogens, use the Fungicide Update on page 38 to find a control that best suits your needs.

Dr. Tom Creswell is the disease manager at the Plant and Insect Disease Clinic, North Carolina State University (Raleigh, N.C.).

TOP 10 TIPS FOR COLLECTING A GOOD PLANT SAMPLE

  • Time is money. Don't wait until the problem is widespread to send a sample. Many diseases and insects are easily controlled if detected early.

  • Dead plants tell no tales. Dead, dry or rotten plants are useless for diagnosis. Collect declining but not completely dead ones.

  • More is better. The main problem may be overlooked if you send only one bedding plant or a single branch from a tree or shrub.

  • What you send is what gets diagnosed. Make sure that your sample is representative of the overall problem.

  • Get to the root of the problem. Many problems are related to the roots and soil. Include plenty of the small roots and about a quart of soil when practical. Dig plants rather than pull them up to keep roots intact.

  • A place for everything. If soil gets on the leaves during shipment, it can mask symptoms or even create a “disease” that wasn't present at shipment. Try to leave soil around the roots so they don't dry out. Bag the roots and soil and tie the bag at the main stem. Wrap foliage lightly in newspaper. Pull the bag over the rest of the plant, and tie it loosely over the top of the plant to keep foliage from drying out. Make sure foliage isn't wet before packaging.

  • The devil is in the details. The more you tell the diagnostic lab about the situation the better the diagnosis will be. Give complete information; include the type of plant, species and cultivar names, location, percent of the site that is affected, symptoms of concern, distribution of symptoms, soil type and drainage, and fertilizers or pesticides used recently.

  • Fresher is better. Mail the samples as soon as you can. Store samples in a cooler during collection and prior to shipment. Avoid mailing samples on Fridays. Use next-day delivery if necessary.

  • Fragile, handle with care. You can use padded mailing envelopes for older plants that are not fragile. However, use crush-proof boxes with crumpled newspaper for padding. This method is best, and it is necessary for fragile plants and turf samples.

  • It's not what you know; it's whom you know. Most states have strong extension services associated with a state university. Extension agents and university websites can be a great resource for problem solving.

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