Plant disease has always been an ongoing concern for farmers. Avoiding, preventing, treating and eliminating plant pathogens takes time, money and knowledge. Synthetic chemical controls have become a primary means of battling plant disease, but their efficacy, environmental safety and cost are leading many growers to explore other options in an attempt to reduce reliance on pesticides.
The pathogens that cause disease in vegetable crops are about 40% fungal. Bacteria, nematodes and viruses each contribute to 20% of crop disease, Dr. Steve Rideout, plant pathologist at Virginia Tech, said recently during a webinar for the Virginia Association for Biological Farming. But no matter the pathogen, disease won’t occur unless there is also a susceptible host and a conducive environment.
“Manipulate any one of these items” and disease can’t happen, Rideout said.
Farmers can work to eliminate pathogens, keep the environmental conditions unsavory for disease development and grow less susceptible crops. Practices such as crop rotation, cover cropping, mulching, equipment sanitizing and proper irrigation can keep pathogen levels low. Planting resistant cultivars, planting crops to avoid times of high disease pressure and controlling the growing environment can keep diseases from taking root. Crop protectants can reduce pathogen numbers and prevent diseases from gaining a foothold.
Crop rotation is a primary method to avoid disease perpetuation in the field. By denying any pathogens present their target host, the disease cycle can be broken. Lack of land for adequate rotation is one concern. For high-value crops, foregoing the crop for a year or two in order to avoid pathogen load in the field isn’t economical.
Cover crops can potentially help reduce pathogen presence. Brassicas are known to have suppressive effects on some pathogens, acting as biofumigants for soil-borne diseases.
“It is very important, with brassicas, to utilize them properly,” Rideout said. By planting in autumn, biomass can be optimized. The cover crop can be flail mowed and incorporated into the soil in spring. “This is the method where you get the most of brassicas.”
The isothiocyanates (ITC) produced by brassicas are the active compounds which act as broad spectrum control for soil-borne diseases such as Pythium and fusarium. They’re also effective against pathogenic nematodes. But ITC is not released unless the cell walls of the brassica plants are macerated. And unless the macerated plant debris is incorporated into the soil, the ITC – which is volatile – will remain on the soil surface, where its impact is blunted, Rideout explained.
ITC is the natural equivalent of metam sodium, a soil fumigant. Not all brassicas have the same levels of ITC. Brassicas bred specifically for an ITC cover crop, such as “Caliente” arugula, can be purchased.
Sunn hemp and the Cahaba White vetch cultivar can inhibit pathogenic root-knot nematodes in vegetable crops. A cover crop of rye or wheat can be suppressive of many vegetable diseases and these grains have very few pathogens of their own that can be transmitted to vegetables.
Inoculum-reducing mulches can be used to prevent splashing during irrigation or rain events so pathogens can’t spread. However, there are some pathogens (particularly Southern blight) which thrive in wood chips. Mulching is very effective against Septoria leaf spot and early blight, however.
“You have to know your pathogen and what will work on it” prior to taking action, Rideout said.
Field sanitation – incorporating plant debris into the soils, disinfecting equipment and working infected crops last – also lowers disease pressures by avoiding the introduction of inoculum from contaminated soils into pathogen-free ones. Rideout noted that equipment purchased from the southern U.S. can, due to the higher disease pressures in that region, be a source of contamination if not properly sanitized prior to use.
Anaerobic soil disinfectant is a newer method of controlling diseases. It involves adding a carbon source, such as cover crop residue, and incorporating it into the soil. Then a plastic or biodegradable mulch is put in place. The third step, which makes this an anaerobic environment, is to water the soil to saturation for two to three weeks.
“It’s very similar to a form of fumigation. You completely alter your microbial communities and really take out your bad pathogens,” Rideout said. This method works on microbes and on weeds. Trials at Virginia Tech with nutsedge-infested fields have shown good success. Other researchers have had positive results with pathogenic nematodes as well as a variety of plant diseases.
Timing your planting to avoid periods of high disease pressure can be another method to reduce disease. Cucurbits are susceptible to downy mildew, which arrives in the Mid-Atlantic in August. Planting cucumbers in spring, for an early crop, avoids downy mildew, as the disease does not overwinter in cold weather regions and is not active until it is reintroduced.
Managing moisture – by not planting when soils are saturated, not over-watering and avoiding crowded plants – can stop the spread of many diseases which thrive in humid, moist environments. Dense plant canopies, which can also be caused by over-fertilization, collect humidity and encourage disease proliferation.
Purchasing (or growing your own) disease-free planting material, whether seeds or transplants, is imperative. Utilizing resistant cultivars is an important way of reducing disease, particularly when viral diseases are of concern. There may be trade-offs with fruit characteristics, yield or maturity dates.
“We don’t have virucides,” Rideout said, and important viral diseases such as basil downy mildew, tomato spotted wilt and Alternaria leaf spot of broccoli are best controlled via cultivar resistance. In recent trials, “Imperial” broccoli showed excellent resistance to Alternaria, while other common cultivars had 95% susceptibility.
Grafting is now being used in vegetables to impart disease resistance into desirable cultivars, primarily with solanaceous and cucurbit crops, Rideout said. Research the traits you need to ensure you’re using appropriate rootstock when grafting.
Pesticides, whether OMRI listed or not, work in several ways to prevent disease. They can be protectant, covering the plant but not entering it, acting as a barrier to disease entry. They can be systemic and enter into the plant and travel through it to impart resistance. There are few antibiotic types left in use, as these have been phased out; they lead to the development of resistant strains of bacteria.
Biological control products are increasingly popular. But many need time to work, as well as repeated applications, in order to “build up a suppressive population,” Rideout said. Whether using chemical or biological control products, “the label is the law,” and should be followed exactly.
Unfortunately, there are new vegetable diseases on the horizon. Tomato brown rugose fruit virus is now in Mexico and has made its way into supermarkets and greenhouses in the U.S., but not yet into fields.
With climate change, increased temperatures can introduce new pest pressures – and new diseases – into regions. Increases in wet weather, storm events or cloudy days can all contribute to making the environment more suitable for a variety of pathogens.
Growers can work to acquire a toolbox of integrated approaches to disease control. By relying on prevention and avoidance through cultural and environmental mitigation techniques, farmers will be thwarting pathogens every step of the way.