As the potato harvest season advances, late-season tuber diseases are of concern. Two diseases in particular—black dot (Colletotrichum coccodes) and silver scurf (Helminthosporium solani)—can diminish tuber quality, reduce storability, and lead to post-harvest losses. Managing these diseases requires careful attention in the late season, as cultural and harvest practices can influence disease.
Black dot
Black dot is caused by a soil-, seed-, and debris-borne fungus that can infect roots, stolons, stems, and tubers. The pathogen is a contributor to the early die complex and is typically enhanced by stress conditions, including temperature and moisture extremes. As a result, black dot causes a reduction in yield and tuber size, and the pathogen causes tuber blemish, reducing marketability and storability. Symptoms on tubers appear as small, black, pinhead-sized structures on the skin surface.
Colletotrichum coccodes builds up in the soil with every potato (or other host crop) rotation and can survive for many years. Knowledge of cultivar resistance is limited to the observations of researchers, growers, and agronomists, however, it is generally understood that thinner-skinned cultivars have higher susceptibility to black dot than thicker or russet-skinned cultivars. Soil fumigation can reduce the quantity of the black dot pathogen in soils, but it is not favored as a management tactic by many, including growers, due to cost and potential negative environmental impact. Some fungicide applications reduce the effect of the stem or early die phase of the disease. The application of the QoI fungicide azoxystrobin in-furrow or prior to first-hilling can reduce the incidence of black dot on daughter tubers. Multiple factors such as soil inoculum level and character, and cultivar susceptibility can influence the efficacy of in-furrow azoxystrobin application.
Some research suggests that the longer tubers are exposed to soil infested with the black dot pathogen, the more severe the black dot symptoms on the tubers. Black dot tuber lesions appear at both the stolon end and arbitrarily around the tuber, suggesting multiple potential infection points from systemic plant infections and via direct tuber contact with the pathogen in the soil. It is generally understood that new tuber infections do not occur during storage, but increases in blemish severity are commonly observed. Black dot tuber blemish is very difficult to distinguish from silver scurf and other skin abnormalities, so much so, that the USDA lumps these into a single category of external discoloration. This makes estimating losses to the disease extremely challenging.
Late-season management tips: 1) Crop rotation: Fields with a history of black dot should ideally rotate out of potatoes for 3+ years, avoiding solanaceous hosts. 2) Irrigation management: Over-irrigation and prolonged soil wetness favor disease development. As vines decline, avoid excess water. 3) Desiccation timing: Prompt vine kill after natural senescence limits inoculum transfer from vines to tubers. 4) Harvest timing: Avoid long intervals between vine kill and harvest, which increases infection risk.
Silver scurf
Silver scurf is a fungal blemish disease of potato tubers caused by the fungus Helminthosporium solani. The pathogen causes tan-to-gray circular lesions typically initiating on the stem-end of the tuber surface, often appearing shiny and silver when wet. Infection reduces both the visual appeal and quality of potato tubers, as the pathogen causes damage to the periderm or skin which enables the onset of other pathogens or enhanced desiccation. This disease is primarily a concern for stored, commercial fresh market potatoes, but the disease can also render potatoes for processing (ie: potato chips) less useful due to increased depth of peeling to remove blemish damage. Additionally, the resulting reduced moisture in potato tubers can impact frying quality. Silver scurf can co-occur with and/or sometimes be misidentified as another fungal potato disease, black dot, caused by Colletotrichum coccodes.
Primary Source: Infested soil, debris-borne, infected seed potatoes, infected tubers in storage
Spread: Rain or irrigation washes spores through infested soil during initial spread. Once in storage, warm temperatures and high humidity allow sporulation to occur. Airborne spores can then infect new stored tubers.
Favorable Conditions: High humidity ( > 90%), high temperatures
Disease Cycle. The silver scurf disease cycle begins with primary infection, which can occur when overwintered spores (conidia) are washed through infested soil and plant debris and onto tubers. Other sources of primary inoculum are infected seed tubers, which can spread the pathogen to daughter tubers. The pathogen infects and causes lesions on the tuber, which can produce more conidia that are released into the soil. At the end of the growing season, H. solani conidia can overwinter in soil, and fungal parts can subsist in soilborne crop debris for several years. Once harvested and in storage, the lesions on infected tubers can enlarge and again produce spores (sporulate) in moderate temperatures and high humidity. These spores can then spread and cause secondary infection in stored tubers.
Late-season management tips: 1) Seed health: Planting certified seed without pathogen is the best management. The use of seed-applied or in-furrow fungicides can reduce the silver scurf pathogen in the potato crop. 2) Harvest promptly: Delayed harvest after vine kill gives the pathogen more opportunity to infect tubers in the soil. 3) Sanitation: Clean storage facilities thoroughly to remove residual inoculum. 4) Storage management: Cool tubers quickly, good airflow, maintain high humidity without free moisture, and store at recommended temperatures to slow disease development. 5) Post-harvest fungicides: can be used to reduce viability of the silver scurf pathogen to limit progress and spread of the disease. Some fungicides with documented efficacy include salts of phosphorous acids (ie: Phostrol) applied post-harvest, and Stadium (Syngenta; pre-mix of azoxystrobin, fludioxonil, and difenoconazole).
Shared Management for Both Diseases includes: 1) limit time between vine kill and harvest: Both black dot and silver scurf thrive when tubers remain in warm, moist soil after vines are killed. 2) Handle tubers carefully: Wounds and bruises increase disease entry points and speed blemish development in storage. 3) Monitor storability: Keep close watch on lots destined for longer-term storage, as these disease symptoms can exacerbate on tubers over time. Silver scurf can spread and create new infections. Black dot is not known to spread via spores in storage, but symptoms can worsen. While these diseases may not be as dramatic as late blight or early dying, their subtle but persistent effects can significantly impact storage success and market returns.