The good news for North Carolina sweet potato growers is most of the fungal diseases that impact the crop can be managed through cultural practices, which will often eliminate the need for fungicides.
One positive of the ongoing battle with the guava root knot nematode is that sweet potatoes will likely see a reduction in the level of soil borne pathogens because farmers are going to be more mindful of using clean seed when planting to combat the nematode, notes Dr. Lina Quesada-Ocampo, a plant pathologist at North Carolina State University.
“There is going to be a culture shift about seed usage now that we have the quarantine for the guava root knot nematode,” Quesada-Ocampo said at the North Carolina Sweet Potato Field Day Oct. 3 at the Cunningham Research Station in Kinston.
The guava root knot nematode has been found on sweet potato farms in eight counties in eastern North Carolina prompting the North Carolina Department of Agriculture to issue an internal quarantine of the invasive pest last October in all 100 of the state’s counties.
The quarantine, which remains in effect one year later, requires sweet potato seed roots to be inspected and certified free of guava root knot nematode before leaving the state. Sweet potato slips are required to have no soil or roots on them before they can leave the state. The quarantine does not affect fresh sweet potatoes leaving the state.
Quesada-Ocampo said managing fungal disease in sweet potatoes requires the right cultural practices. It begins with being mindful of site selection for the crop. She urged farmers not to plant in fields that have a really bad history of soil borne pathogens and to always use clean and certified seed if possible.
“Also, be really careful with slip cutting. Make sure you are cutting your slips at least one inch above the soil level. Crop rotation is important. We recommend three years away from sweet potatoes. This works well for most of our soil-borne fungal pathogens so keep this up.” Quesada-Ocampo said.
In addition, controlling weeds and insects is critical for avoiding soil-borne pathogens in sweet potatoes.
“Insects can wound the root and become an entry point for soil-borne pathogens. Weeds can harbor some of the soil-borne pathogens. For example, morningglories become infected with the black rot pathogen, that is a way the pathogen can survive so weed control is really important for disease control also,” she said.
“As much as possible, although I know it is hard, try to avoid wounding the roots during harvest because any wound is going to become and open door for a plant pathogen,” she added.
At post-harvest, Quesada-Ocampo said it is important to cure sweet potato roots immediately because a lot of the disease issues occur at post-harvest because growers haven’t done a good job of curing. At the packing house, regularly sanitizing your equipment is a must.
“The black rot pathogen is very good about spreading in water and sticking to equipment. The spores it produces are really sticky. if you are not sanitizing your packing house and equipment regularly, you’re going to have that pathogen everywhere,” she said.
“Try to rely on fungicide application at packing. It is really important to protect those roots that will ship for the domestic or export markets.”
Quesada-Ocampo pointed out that fungicide options for post-harvest control are limited in sweet potatoes.
“Our biggest export market is the European Union, and they are having a lot of recent changes in residue levels allowed. Because of that it has become harder to use fungicides. But we still have some chemistries available that we can use. Most folks are relying on Scholar which can still be used post -harvest. Scholar is really good about controlling Rhizopus, but it is not good for controlling black rot,” she said.
The good news is that earlier this year the fungicide Stadium was registered for sweet potatoes post-harvest. Quesada-Ocampo said the three active ingredients in Stadium — fludioxonil, azoxystrobin and difenoconazole — allow sweet potato farmers to control both black rot and Rhizopus with a single fungicide.
“The active ingredients in Stadium have residue levels established for the EU so it may be an option for exports once we determine residue levels at the labeled rate” she said.