Bulletin #2417, Protecting Potato Tubers from Late Blight
By I. Kutay Ozturk, Extension Potato Pathologist, University of Maine Cooperative Extension
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This bulletin lists the best management practices for minimizing the risk of spreading late blight to tubers during late-stage growing season, harvest, and storage.
Vine-killing
Vine-killing is essential for all potato growers in a late blight season. Late blight requires living host tissue to survive and ensuring that the vines are completely desiccated before harvest is crucial. Green digging, a practice that exposes tubers to pathogens and increases bruises, significantly increases the risk of late blight incidence. Harvesting potatoes is recommended at least two weeks (preferably three weeks) after the vines die down.
Chemical vine-killing or flaming should be preferred over mechanical vine-killing, as the latter can spread the disease and fail to achieve complete desiccation. On infected fields, sulfuric acid can be considered for vine-killing.
Fungicide applications
It is crucial to continue fungicide applications until the vines are entirely desiccated. Products containing triphenyltin hydroxide, such as Agri Tin and Super Tin, kill spores on contact and can be used during vine-kill to reduce tuber infections dramatically. Late-season mancozeb applications before vine-kill may also limit tuber infections. Various fungicides, including chlorothalonil, fluazinam, cymoxanil, and copper-based products, are available to control late blight. Mefenoxam/metalaxyl-containing products can be effective on some strains (Table 1). Please refer to the Potato Pest Control Guide for the most up-to-date list, and always check product labels before use.
Table 1. Overview of Phytophthora infestans strains, mating types, and mefenoxam sensitivities.
| Strain | Mating Type | Mefenoxam/metalaxyl sensitivity |
|---|---|---|
| US-1 | A1 | Sensitive |
| US-6 | A1 | Resistant |
| US-7 | A2 | Resistant |
| US-8 | A2 | Resistant |
| US-11 | A1 | Resistant |
| US-20 | A2 | Sensitive |
| US-21 | A2 | Sensitive |
| US-22 | A2 | Sensitive |
| US-23 | A1 | Sensitive |
| US-24 | A1 | Sensitive |
| US-25 | A2 | Resistant |
Harvest and Storage
Best practices in bruise management are essential to minimize the spread of late blight to tubers since bruises are entry points for pathogens. Vine kill ensures that a good skin set is formed, which reduces bruise risk. Potatoes should be harvested from dry soil when the tuber pulp temperatures are between 45-65 °F.
Treating with phosphite-based fungicides post-harvest can provide additional protection in storage. However, if post-harvest products are used, it is essential to adjust the rate and water volume appropriately following the label recommendations. Too high volumes could cause potatoes to stay wet in storage, creating ideal conditions for other diseases such as bacterial soft rot.
Before storage, tubers need to be graded for late blight since late blight can spread from tuber to tuber. After harvest, it is imperative to cool the potatoes to the curing temperature of 50 °F as quickly as possible. Throughout storage, provide a high ventilation rate and maximize fresh air intake.
If late blight on tubers is suspected, storing the tubers at 38 °F with 85% humidity is ideal. Processing potato growers are recommended to work with their customers to process late blight suspect potatoes as quickly as possible.
References
- Olsen, N., Miller, J. (2015). Checklist for managing late blight approaching harvest and into storage. Miller Research. millerresearch.com/wp-content/uploads/2019/01/Checklist-for-Managing-Late-Blight-Approaching-Harvest-and-Into-Storage-2015.pdf
- Saville, A., Graham, K., Grünwald, N. J., Myers, K., Fry, W. E., Ristaino, J. B. (2015). Fungicide sensitivity of US genotypes of Phytophthora infestans to six oomycete-targeted compounds. Plant Disease, 99, 659-666.
- Larson, E. R., Migliano, L. E., Chen, Y., Gevens, A. J. (2021). Mefenoxam sensitivity in US-8 and US-23 Phytophthora infestans from Wisconsin. Plant Health Progress, 22, 272-280.
- Reid, J. (2014). New Strain of Late Blight Discovered. VegEdge, 14(20). Cornell Cooperative Extension. rvpadmin.cce.cornell.edu/pdf/veg_edge/pdf149_pdf.pdf
- Lacy, M.L., Hammerschmidt, R. (1995). Diseases of Potato: Late Blight. Michigan State University Extension. canr.msu.edu/uploads/files/e1802.pdf
Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.
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