Verticillium wilt is a major soil-borne disease that can result in yield losses as high as 50 per cent in potatoes. The roots of a potato plant, or other host plant, release compounds that stimulate the germination of Verticillium dahliae microsclerotia, the fungus’s survival structures in the soil. The fungus then grows, penetrates the plant’s roots and spreads up the plant, eventually causing wilting, yellowing and early dying in potatoes.
“As Verticillium dahliae completes its life cycle in a plant host, the fungus uses the dying tissue of its host to make more microsclerotia, which are small, darkly pigmented, multicellular and irregularly shaped survival structures,” says Zachary Frederick, who was part of the Washington State University (WSU) research group that conducted the study.
“It can take five to 30 microsclerotia per gram of soil to infect a susceptible potato cultivar. A lot of factors are involved with this threshold, including soil properties, environmental conditions, the host’s susceptibility, and the properties of the fungus itself. One of those properties for the fungus is its aggressiveness to the potato host,” he explains.
“Verticillium that is aggressive on potato generally requires fewer microsclerotia per gram of soil to infect a susceptible potato cultivar. Aggressive Verticillium also manifests as more severe disease later on in the season, which can catch potato growers by surprise because their soil tests show low counts of the fungus.”
This aggressiveness is specific to potato; the fungus acts normally when attacking a wide range of other host species. Frederick notes, “So far, the host-aggressive phenomenon has been well documented in Washington for Verticillium wilt of potato and of mint. Isolates that are aggressive on mint are not aggressive on potato, and vice versa.” Host-aggressiveness has also been documented in cotton and olive, and possibly exists for other crops.
Scientists are still trying to nail down exactly how such host-aggressive isolates develop. It is thought the fungus gradually adapts to the host—each time it completes its life cycle in that particular host species, it becomes more efficient at killing that host, needing less inoculum to cause increasingly severe Verticillium wilt.
The exact number of potato crops needed to turn an ordinary isolate into a potato-aggressive isolate is not yet known. But if it is a gradual process, then what would explain the situations observed by WSU researchers in which a field’s potato crop is fine one year, but when the next potato crop in the rotation is planted a few years later, the crop has severe wilt?
The WSU research group wondered if perhaps another crop species or a weed species could be contributing to development of potato-aggressive isolates in such situations. So, Frederick, who was a PhD student at the time, was assigned the weed study, and David Wheeler, another PhD student, was assigned the crop study.
Searching for a Weed Connection
Working with WSU plant pathologist and professor Dennis Johnson, who leads the research group, and research technologist Tom Cummings, Frederick carried out a greenhouse study with 16 weed species. The 16 species are all found in Washington’s Columbia Basin and also occur in other parts of the United States and Canada.
The study’s first objective was to test the weeds for susceptibility to eight isolates of Verticillium dahliae. “Some of the 16 weed species have been subject to Verticillium research since the late 1960s, so we reconfirmed their susceptibility. Other species did not have a scientific background with Verticillium research because we included grasses in our study. Verticillium is traditionally thought of as a disease of dicots, not monocots, like grasses,” explains Frederick, who is now the applied research agronomist with the Manitoba Horticulture Productivity Enhancement Centre Inc. at Carberry.
The 16 weed species included five nightshades: bittersweet nightshade (Solanum dulcamara), black nightshade (Solanum nigrum), eastern black nightshade (Solanum ptycanthum), hairy nightshade (Solanum physalifolium), and litchi tomato (Solanum sisymbriifolium). Nightshade weeds are cousins of potato (Solanum tuberosum) and are hosts to some other potato pathogens.
The researchers also tested four other broadleaf weeds: annual sowthistle (Sonchus oleraceus), common lambsquarters (Chenopodium album), Powell pigweed (Amaranthus powellii), and tumble pigweed (Amaranthus albus). The study’s grassy weeds were: annual bluegrass (Poa annua), barnyard grass (Echinochloa crus-galli), downy brome (Bromus tectorum), green foxtail (Setaria viridis), large crabgrass (Digitaria sanguinalis), rattail fescue (Vulpia myuros), and wild oat (Avena fatua).
The eight Verticillium dahliae isolates included two from mint, two from tomato,and one each from potato, sugar beet, sunflower and watermelon. The potato isolate was aggressive on potato, and the mint isolates were aggressive on mint.
The second objective was to identify any weed hosts where the aggressive isolates created more disease or more microsclerotia, compared with the ordinary isolates.
Frederick inoculated seedlings of the weeds, testing each isolate on its own in each of the weed species. He
grew the inoculated plants in a potting mix in the greenhouse.
“Research on Verticillium indicates that infection happens prior to flowering for most crops. Once we had as many of the weeds as possible flowering and setting seeds, we held back the water to the plants, allowing them to die slowly over the course of about two weeks. It takes about two weeks for the fungus to form microsclerotia when it senses the host is dying,” says Frederick.
He checked the plants for symptoms of the disease, and he compared the aggressiveness of the different isolates by counting the number of living microsclerotia produced by each weed with each Verticillium isolate.
Every single weed species in the study was host to at least one Verticillium dahliae isolate, producing at least some microsclerotia after inoculation. Fortunately, many of the infected weeds—such as large crabgrass, barnyard grass and rattail fescue—produced only tiny amounts of the microsclerotia.
Initially, the researchers were surprised to find infections in the grassy weeds, so they dug deeper into the scientific literature. “It turns out that since the 1960s, and more so from the 1990s through to about 2015, there have been a fair number of publications where barley, rye, wheat and even corn have all been reported as Verticillium hosts, generally without any symptoms of the disease. And in one very unique case, a researcher found a barley plant that actually displayed symptoms of Verticillium wilt,” says Frederick.
In Frederick’s study, none of the infected weeds—no matter whether they were nightshades, other broadleafs, or grasses—had any visible symptoms of the disease. “There was no stunting, no yellowing, no wilting—none of the hallmarks of the disease,” he notes. “[For instance] a six-foot tall wild oat plant [grown in a greenhouse] had a systemic Verticillium infection throughout the entire plant, but had no outward signs that the fungus was present.”
The infection in that six-foot wild oat plant was caused by an unusual isolate from tomato, which produced a lot more microsclerotia on wild oat than any of the other isolates. That odd isolate also produced infections with significantly greater numbers of microsclerotia in a few of the other weeds in some of the trials.
The study reconfirmed that nightshade weeds are susceptible to Verticillium. “The nightshades in general were very susceptible to Verticillium, and their infections produced lots of microsclerotia. In the case of species like hairy nightshade and eastern black nightshade, the potato-aggressive isolate didn’t produce more microsclerotia than the rest of the isolates, so the aggressiveness wasn’t maintained, but these weeds were highly susceptible to virtually every Verticillium isolate that we tested and returned a great deal of microsclerotia to the soil,” says Frederick.
Only black nightshade maintained the aggressiveness of the potato-aggressive isolate. “Like the other nightshades, black nightshade was very susceptible to Verticillium, but the potato-aggressive isolate produced more microsclerotia on black nightshade than the other isolates.”
“A big take-home message from this study is to tie together your weed management with your Verticillium wilt disease management,” says Frederick.
Controlling nightshades is especially important. “Based on this greenhouse study, virtually all nightshades are very good Verticillium hosts. If a nightshade weed is infected, microsclerotia will be produced, and where the plant residue breaks down in the field is where you’ll most likely see more microsclerotia returned to the soil. So, where you have these nightshades, yourVerticillium problem could possibly continue to increase.” And if you have black nightshade, you might get even greater disease pressure due to potato-aggressive Verticillium dahliae.
Another tip from Frederick is to control weeds while they are still small. “Don’t give weeds the opportunity to be hosts to Verticillium and especially to complete their life cycles. Typically, it takes about two weeks of the weed being in the presence of Verticillium in the soil for the fungus to find and infect that weed. When the weed is small there is low potential for microsclerotia. That six-foot wild oat plant had a lot of plant mass to play host to Verticillium whereas a three-inch wild oat that is killed off at that stage would return less Verticillium to the soil.”
Also, remember that infected weeds will likely have no symptoms even if they have a severe infection that generates lots of microsclerotia.
Frederick recommends making weed control part of an integrated, long-term approach to managing Verticillium wilt in potato. “The fungus takes years if not decades to build up in the soil to the point where it becomes a noticeable problem. So there is a lot of room for proactive treatment ahead of time, and then measures such as chemical fumigationmay not be needed. Although weed management isn’t a silver bullet for dealing with Verticillium dahliae, it fits into this larger picture for Verticillium control.”
Another important long-term strategy is to avoid very short potato rotations, especially back-to-back potato crops, to reduce the risk of developing potato-aggressive Verticillium. More information on cropping options for Verticillium management could come from Wheeler’s study, which will be completed this fall.