Is it a virus disease or something else? A few pointers
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
This is the time of year that virus symptoms become apparent, particularly in blueberries. A cool spring and slow growth also brings out symptoms more than warm weather. Virus and virus-like diseases in plants manifest themselves in several ways and can be easily confused with abiotic disorders like nutrient deficiencies and herbicide injury. Viruses are tiny pathogens that are only visible with an electron microscope. Their simple make-up, a strand of genetic material covered by a protein coat, belies the damage they can cause. Think of viruses as miniscule hijackers of plant cells. Once they enter a plant cell, they force that cell to multiply the virus particles and thereby derail the cell’s normal functions. Once a plant becomes infected, the virus spreads throughout the plant tissues including the roots. This is called a systemic infection, and the plant is infected for life. This also means that you cannot cut out the symptomatic plant parts and expect the plant to be cured.
Symptoms that may indicate a virus disease include: yellowing or reddening of leaves, mosaic or mottling, crinkling or malformation of leaves and other plant organs, stunting, poor fruit set, generally poor growth, and plant death. Some of these symptoms can also occur when the plants are malnourished or the root system is compromised (e.g., by root rot or nematode damage). The recent cold weather has also induced some reddening in plants due to limitations on nutrient uptake. Symptoms caused by herbicides, such as Round-Up or 2,4-D may also include stunting and malformation of leaves: 2,4-D is especially tricky because it can volatilize and affect plants some distance away from the application site. Herbicide drift from another crop (e.g., Dicamba from corn or soybeans) can also affect nearby non-target crops. Here are a few pointers that may help you decide whether symptoms that you are seeing may be due to a virus or another cause:
- Are the symptoms present in a few scattered plants or in many plants over a large area? If present uniformly over a large area and also affecting weeds, it is more likely to be caused by herbicide injury, some nutritional disorder or soil condition. Have soil and plant tissue analyzed for nutrients. Some herbicide residues can be detected if samples are taken soon after the application or drift occurred but this is rather expensive.
- Did the symptoms show up suddenly or have they been worsening over the season or the past couple of years? If the symptoms showed up suddenly in an otherwise normal year, herbicide injury is a possibility, especially if many plants are affected. Symptoms caused by virus diseases usually worsen over time.
- Does the disease seem to be spreading? If so, it may be a virus disease. Virus diseases vectored by nematodes usually spread in a more-or-less circular pattern in a field, whereas viruses vectored by aphids spread more readily down the row.
- Did you apply Round-Up last fall? This herbicide may get transported into the roots and may not show symptoms until the following spring.
- Are symptoms showing here and there in a newly planted field? Consider your source of plants. Did you buy virus-tested planting stock? If not, you may have imported a virus disease with the planting material.
- If you suspect a virus disease, send a plant sample to the MSU Plant Diagnostic Lab. The best time to send samples in is in the spring, when young plant tissues (leaves or flowers) are most likely to contain virus particles. Samples have to be fresh for virus indexing, so send them on ice in express mail or hand-deliver the cooled sample.
Just as in people, virus diseases in plants are difficult if not impossible to cure, so prevention is the best method of control. Buy virus-tested planting stock whenever possible: it is definitely worth the investment, especially when planting a perennial crop. If you see suspicious symptoms, rogue out and destroy affected plants quickly before they become a source of inoculum and practice good insect control. When planting into a field that had a nematode-transmitted virus disease previously, fumigate the soil before replanting or grow non-host cover crops (e.g., rye or mustard) for a couple of years.
Dr. Schilder’s work is funded in part by MSU‘s AgBioResearch.