We often treat disease as an exception, a departure from the ordinary state of things. Thanks to public health infrastructure such as sewage treatment, restaurant inspections, and ventilation standards, we live disease-free for long intervals and don’t spend our all waking moments bracing for the cough that could signal misery, isolation, or death. Public health officials admonish us to cover our cough, wash our hands, get vaccinated, in short, to alter our behavior for the active prevention of disease, which is a menace that is always with us. However, not every employer offers sick leave, not every public gathering place supplies tissues, and not every medicine cabinet is stocked with unexpired analgesic, measures that can suddenly become necessary when the the darker side of life inevitably arises.
The situation is similar for agricultural producers. They prepare the soil, select the varieties, do the planting, manage water and fertility, weed, harvest, market the product, pay the workers and the creditors, and more. It would be understandable for them to relegate thoughts of disease to the back of their mind, especially with diseases they have no firsthand experience with. Disease is a chance event, and growers often get lucky. However, they ignore disease prevention at their peril.
Professor Cassandra Swett is a plant pathologist who bemoans the inattention that agronomists give to plant disease. Among other endeavors, Dr. Swett has lent her expertise to the study of deficit irrigation, an emerging practice intended to address water scarcity. The idea is to reduce water inputs only low enough to slightly stress the plants, not to reduce yield. In theory, plants can compensate for a certain amount of environmental stress. However, Dr. Swett’s first question is always about plant disease, and she is quick to point out that stress can often exacerbate disease, or even trigger symptoms in apparently healthy plants. Before she began asking about deficit irrigation and disease, none of the deficit irrigation proponents had considered that question.
Many fruit crops produce better flavor when they receive less than plentiful water. Tomatoes are one example, and thus a logical choice for deficit irrigation trials. Dr. Swett found that at light levels of deficit the plants had more infection with the fungus Fusarium than they would have under conventional water management, while at a greater deficit they had less infection. This finding of an apparently paradoxical result is also reflected in work her lab did with greenhouse poinsettias, where deficit irrigation reduced populations of water molds, but wherever the water mold organism Pythium was present it caused more disease. These are just the initial explorations of disease dynamics under deficit irrigation, but without Dr. Swett’s contribution, agronomists might have made sweeping recommendations that would have saved some growers money while costing other growers in terms of yield.
Dr. Kamyar Aram is a scientist at the Foundation Plant Service of UC Davis, the facility charged with providing virus-free grapevine propagation material to the nurseries that growers depend on for their supply. As part of his public outreach, Dr. Aram receives calls from California growers when their vineyard is overcome with viruses, at a point when little can be done to save production. Dr. Aram laments that growers generally do not regularly scout for virus symptoms and rogue symptomatic vines, that is, rip out infected individuals before the infection can spread and interfere with yields across the entire vineyard, or in terms of the enterprise, let go of underperforming assets before they become toxic across a wider portion of the portfolio.
The grapevine viruses causing the biggest dent in California wine production are the Grapevine Leafroll-associated Virus strain 3 and the Grapevine Red Blotch-associated Virus (“GLRaV-3” and “GRBaV”, following to the conventions of virus nomenclature). Grapevine viruses are transmitted through grafting infected material onto a new plant or through feeding and dispersal by sucking insects, such as mealybugs for GLRaV-3 or the three-cornered alfalfa hopper for GRBaV. Different virus strains presumably arose from an ancestral virus by coevolving in the grapevine population of a locality. Coevolved grapevines may tolerate their local virus, but a globalized grape-growing industry has spread viruses from every part of the grapevine’s native range to all the major wine regions. Such a situation can lead to viral infection of naive vines, mixed infections where viruses have synergistic effects on their host, and trading of virulence genes to create souped-up viruses.
Once a grapevine gets infected with a virus, it stays infected with that virus until the end of its life. Yield can be reduced, sugar production is impaired, and wine quality suffers. Vineyards that catch the eye with stunning red foliage in the fall are showing off virus infection; the vines’ natural fall color is yellow.
Grapevine Leafroll-associated Virus strain 3 is originally from Israel. Some traveler decided it was more important to get his hands on a particular grapevine for his vineyard than to protect the wine industry of an entire region. With the recent invasion of the vine mealybug into California, GLRaV-3 has a more efficient means of transport to move from vineyard to vineyard. The presence of the vine mealybug means that if you are a grape grower, your neighbor’s infected vineyard has become your affair. If your neighbor rips out his infected vineyard and replants, mealybugs can reintroduce virus from his neighbors. And because vine mealybugs can hide out on grapevine roots in certain soil types, a grower’s own soil can be a source of future virus infection.
Dr. Aram posits that growers in the Lodi area of California could eliminate GLRaV-3 for good if they all ripped out their vineyards simultaneously and kept the region grapevine-free for a season, and then re-planted with clean material. As far-fetched as this scenario might sound, it has been done in New Zealand and South Africa. In the US the emphasis on individual prerogative over collective action, codified in the economic system, generally makes thoughts of region-wide cooperative efforts inconceivable. Business as usual insures that GLRaV-3 will continue to be a problem.
Other examples abound. Dr. Honour McCann reminds us that the New Zealand breeders who developed the golden kiwifruit did so without considering the bacterial disease known as Psa that was attacking green kiwifruit across Asia at the time. This oversight caused the disease to spread on susceptible vines into New Zealand, Australia, and Europe, where treatment options are limited, and in some cases amount to an IV bag pumping antibiotic into each vine. An example from my own experience is the way some small growers will buy seeds from the bulk bin at the grocery store for planting. They save money on the purchase price of the seeds, but they don’t realize that unlike commercial seeds for planting, seeds marketed as food are not required to be produced on disease-free soil, they do not receive hot water or other treatments to kill seedborne pathogens, nor are they tested for pathogens. These growers are risking not just this year’s yield, but also the health of the field they plant in.
What should be done? Plant disease is an integral part of agriculture. All people involved in the agricultural system, from germplasm prospectors to breeders to agronomists to seed producers to growers to laborers to consumers should take a moment to inquire about the plant diseases to watch out for in their practices. Asking about plant disease should be as routine as washing your hands. Plant pathologists are standing by to take your call. As the American Phytopathological Society’s popular T-shirt says, “Don’t get caught with your plants down!”