 |
|
||||||||||||||||||||||||||||||
 |
 |
||||||||||||||||||||||||||||||
|
|
||||||||||||||||||||||||||||||
|
|
||||||||||||||||||||||||||||||
Karen Delahaut, Outreach Specialist, UW-Madison
You’ve just been monitoring your plants and notice that some of the plants don’t look quite right but you’re not sure what’s causing the problem. What should you do to try and figure out what’s going on?
Diagnosis of plant problems is both an art and a science. Proper diagnosis often requires extensive knowledge and continuous exposure to problems commonly associated with the species of plants you grow.
The first rule in making a diagnosis is to know your plants. You must be familiar with the common characteristics of each species and cultivar you grow. You should also know what the cultural requirements are for the various plants. Finally, familiarize yourself with the key pests for each crop. It’s not necessary to learn every obscure disease and insect pest that has ever been identified on a particular species but know the common pests.
When faced with a symptomatic plant, it is important to keep an open mind and not rule out any possibility prematurely. If more than one plant is affected, you must determine the extent of the problem. Examine the area and check your records of the environmental conditions and cultural practices to which the crop has been exposed. Are the affected plants all the same variety, species, family? Rarely do insects and plant pathogens cross family lines. Widespread problems are likely to be due to cultural practices or adverse environmental conditions in the greenhouse.
The next step is to take a closer look at the affected plants. Do you see any insects or cast skins on the foliage, buds, flowers, or stems? Are there any sporulating lesions or is there a “fuzz” of mycelium covering the affected plant part indicating there might be a fungal pathogen at work? Remove a plant from its pot and examine the roots. Foliar symptoms do not necessarily indicate that the cause is on the foliage – it may be the result of a root rot or soil-borne insect. Are the roots light in color and firm, well-developed and free of any soil insects?
How are the affected plants arranged in the greenhouse? Are they scattered all over the bench? Or are they all near a window, vent, or door? In a hoop house, are the affected plants beneath the support beams?
Did the symptoms appear suddenly or gradually over a period of weeks? Rarely are biological causes associated with a sudden onset of symptoms. Have herbicides been applied outside of the greenhouse? What pesticides have been applied? The symptoms could be an indication of phytotoxicity. Have growth regulators been used?
Once you have a clear understanding of the history of the planting, the symptoms expressed, and the extent of the problem, you can begin ruling out unlikely causes. Begin by eliminating the obvious. If the symptoms developed overnight and you are unable to recognize any signs of disease or insect activity, it’s probably safe to rule out a biotic cause. If you can rule out the most severe potential causes for the symptoms expressed, it may not be necessary to determine the exact cause. Consult reference for descriptions of pests or diseases with which you may be unfamiliar.
If you still can’t identify the problem, seek help. Your county cooperative Extension office is your first stop. If they are unfamiliar with the problem, they will send a sample to the Plant Disease Detection Clinic or the Insect Diagnostic Clinic in Madison for identification of the cause and recommended treatments. Keep in mind that a laboratory examination doesn’t replace an accurate and thorough field examination and the more information you can provide the diagnostician the more likely (s)he will be able to help you.
By systematically implementing the above suggestions, you will begin to improve your diagnostic skills. Keep in mind that you cannot become an expert over night. It often takes years of experience to be able to readily identify pest problems on sight.
