Organic spinach has a big problem — downy mildew. Conventional growers have effective sprays for it, but organic growers rely largely on resistant spinach varieties. Unfortunately, new races of downy mildew that overcome spinach resistance genes are appearing every year.
Downy mildew is not a true fungus, but an oomycete, a relative of the potato late blight pathogen, the one involved in the Irish potato famine. There are hundreds of species of downy mildew, each specialized on one or just a few host plant species. Spinach downy mildew is not known to infect any other plant species besides spinach. The pathogen grows inside the leaves, causing them to yellow and wilt, and then it emerges from the stomates on the underside, giving a characteristic downy appearance. The soft downy portion produces spore-bearing structures anatomically characterized as sporangia (singular sporangium) that themselves break off and float through the air, acting as individual spores to propagate the pathogen.
Downy mildew is a problem not just for its direct effect on spinach leaves. Because organic spinach from California once had an instance of E. coli contamination that sickened many consumers, killing three, any infection of spinach is looked upon with suspicion by wholesale buyers, even that of a pathogen that cannot infect any plant but spinach, much less a human. Thus the threshold for rejection of an entire field of spinach is 1-5% infection with downy mildew. This low rate may help reduce the chances that a clamshell of spinach will turn to mush, but it makes organic spinach production a risky endeavor.
Dr. Robin Choudhury did his dissertation research examining the disease dynamics of spinach downy mildew. He studied aerial spore (sporangium) dispersal by means of traps with small sticky rods revolving around a pivot that capture everything floating through the air. Traps were located at points up and down California’s Salinas Valley, and the rods were changed out every few days during the course of two seasons. By extracting the DNA from everything stuck on the rods, Choudhury could test for the presence of downy mildew spores using a highly sensitive, highly specific technique known as qPCR. This technique amplifies a segment of DNA unique to the species of downy mildew from spinach, Peronospora effusa. By monitoring the amount of amplification necessary to detect the specific DNA segment, a researcher can also determine the number of spores captured.
With volumes of data on hand, Choudhury applied epidemiological analyses to find patterns. He found that the number of captured spores showed an exponential increase over each season. Furthermore, within that overall trend there were regular pulses of spores that may have corresponded either to pathogen generation time or to spinach harvest cycles. Either way, ever-present spores are bad news for growers.
In another sleight of analysis, Choudhury estimated the dispersal distance of the spores from an outbreak. Of course the farther a spinach field is from an outbreak, the fewer spores will arrive at it. The question is how far the spores will travel. According to Choudhury’s models, there is some likelihood that there is a limit to the distance that the vast majority of spores will travel, but there is a higher likelihood that a significant proportion of spores will travel very long distances, potentially endangering spinach fields in remote locations in the valley. These data were rather noisy, though, and did not completely rule out the former scenario. On the ground, Choudhury showed that an active outbreak would lead to a higher level of infection 5.6 meters away, but beyond that the infection rate was no different from the background rate.
Dr. Choudhury also did a trial of mixed varieties of spinach. The reason for mixing varieties with resistance to different races of a pathogen is twofold. For strictly market reasons, if one pathogen race takes out a variety, the remaining resistant varieties will still produce. A grower never knows which races will be present, and so will have to grow multiple varieties to insure a harvest. However, the current practice is to grow the different varieties separately in wide strips because the growers worry about inconsistencies in leaf traits. This arrangement misses out on the epidemiological benefit of growing the mix of varieties in a single bed.
If a field is full of plants that are genetically identical, then a single infected plant will give off spores that are adapted to every plant in all directions. However, with a good mix of varieties, the plant’s neighbors may all be resistant to those spores, and any spores that land on resistant neighbors will be subtracted from the total pool of spores available to spread the disease. This dilution of susceptible hosts can slow the spread of disease. The intercepted spores may even pre-arm the neighboring plants against races of the pathogen that are adapted to them, a phenomenon known as induced resistance.
Dr. Choudhury showed the growers his mixed-variety field and challenged them to point out non-uniform plants; they could not. Unfortunately, the trial was disappointing. Although few varieties succumbed to downy mildew, resulting in a lower-than-expected total disease incidence for a mix, the disease incidence still crossed the 1-5% threshold. It may be that there have already arisen too many pathogen races for this method to succeed. The only scenario where mixed-variety plantings might work would be if the method were widely adopted across the valley.
The reason that new races continue to emerge is that growers depend too heavily on a small number of varieties that are resistant to the known races. With the vast acreage of the Salinas Valley planted to just a few varieties, the fast evolution of the pathogen will generate races that are pathogenic on these varieties as well. One strategy to put the brakes on this fast evolution would be to discard seed lots with too many sexual spores of the pathogen. This type of spore is where a new genotype can be formed, and with enough new genotypes, a new pathogenic race can arise. However, the seed producers, who are located in states to the north, might not be willing to discard a portion of their product for the benefit of the Salinas Valley system.
A couple of practices that Choudhury was not able to challenge were planting density and irrigation. The demand for organic lettuce has burgeoned, and now the standard practice is to grow a high-density lawn of spinach on 80-inch beds that stretch off into the horizon. Spinach production fills a huge area of the Salinas Valley, nicknamed the nation’s salad bowl. The endless expanse of spinach is a giant petri dish for pathogens, and the crowding insures a continuously humid layer that promotes spore germination. To make matters worse, the growers overhead-irrigate, absolutely insuring excellent spore germination conditions.
Big organic growers have been lured by the profit motive to shift to an industrial mode of spinach production without regard to agroecological realities. In so doing, they created a downy mildew crisis, and they have called in the plant pathologists to deal with the problem. It makes one wonder how much of what plant pathologists do is simply trying to clean up the disasters of industrial agriculture. Large-scale organic spinach is just as industrial as conventional agriculture. Resistant varieties, perfectly timed biofungicide application, disease scouting, and hand roguing might get this season’s crop to market, but without an agroecologically sound system design, these measures may not save organic spinach production into the future.