Crops and Cropping System Division


Plant disease and Pest Diagnostics

The process of diagnosing plant problems involves a systematic approach by which a series of asking questions, observation of the surrounding environment, and good record keeping which can decide whether the problem is due to an insect, pathogen or other factors like phytotoxicity from applied chemicals.

Q: What tools are needed to assist with the diagnosis?

A: When observing the said plant in question, one must use items like a compact shovel to dig up plants without harming the root system or collect soil samples, use of notebooks so you can refer to thoughts at a later time, a knife to dissect the plant sample for further investigation, a vial or container to put samples into and a camera among others to document the signs and symptoms of the plant. 

Q: What is the host plant?

A: The proper identification of the host plant species will generally provide adequate information to rule out other insect and pathogen species that does not infect the said plant. 

Q: What is normal for the plant?

A: Understanding and recognizing the appearance of the plant species will help to determine if the problem exist or not. By knowing the cultivars agromorphological characteristics comparing the affected plant to a healthy one of the same growth stage will help to determine the abnormalities. 

Q: What is the pattern of the damage?

A: Plant injury can be uniform or non-uniform depending on the problem. If the injury can be randomly seen in the total of the field, it can often indicate a plant disease. On the other hand, if the injury can be viewed in uniformly in the plant population, this could often indicate an abiotic factor like nutrient deficiency. If different plant species are affected, it usually indicates an abiotic disorder and not a biotic disorder, as living organisms have a limited host range.

Q: What are the symptoms and do you see any signs?

A: The fastest way to identify an abiotic disorder is to observe signs of insect, pathogen or herbivore. The easiest way to tell if its caused by insects is to observe damage like insect chewing, bored holes or rasped plant tissues. As for signs of pathogen, fungal strands, fungal fruiting bodies, bacterial ooze, and mold or mildew. The symptoms on the other can be caused by mechanical injury or physical injury like broken stems, punctured leaves, uprooted plants or bruised fruits. Look and observe the entirety of the plant to assess its signs and symptoms.

Q: What are the site conditions?

A: Plants thrive the best in its favorable condition. Plants grown in poorly maintained soil, inadequate sunlight, water and soil nutrients will grow poorly. Knowledge of the optimal growing condition of the plant will help narrow down the cause of the injury. Previous records of the site can also be useful especially if the site has been affected by pathogens or been infested by insects prior to the usage.

Q: What do surrounding plants look like?

A: If other plant species in the surrounding area also exhibit the symptoms of the injured plant, it is likely that the root cause of the problem is due to environmental factors such as abrupt changes in weather conditions, extreme temperature, excessive rainfall or draught, strong winds and the likes.

Q: What cultural management practices were used to grow the plant?

A: Knowing what the practices used to grow and maintain the crop can provide an insight on the problem at hand. The problem might be due to over fertilizer usage of nutrient deficiencies which makes the plant more susceptible to pathogen or insect feeding. Irrigation as well can be a contributing factor in terms of how long the crop was irrigated, where the source is, or whether the water was previously contaminated by chemicals from spraying. Another is whether pesticides were applied to the plant itself or the surrounding plants. The kind of pesticides, rate and how it was applied can be the cause of the symptoms of the injury.

Q: What resources are available to help with the diagnosis?

A: Resources like reference books, websites and social media sites can help in the identification of pest and disease. It is always advisable to check whether the source of information is from a credible source. Further diagnostics can be done by consulting an entomologists or plant pathologist when it comes to insect pests and diseases. 

Derived from the Plant Disease Facts by the Department of Plant Pathology and Crop Physiology, LSU AgCenter


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