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Potato Mop Top Virus (PMTV) and Powdery Scab – Management

As with other potato viruses, the best management method for PMTV is to avoid introduction of the virus or its vector to your fields. Once the virus or the powdery scab vector has been detected, steps must be taken to avoid spreading these organisms to clean fields. The following steps can help to manage this disease.

Avoid introduction of powdery scab and PMTV onto your farm.
  • Don’t plant symptomatic seed. Inspect seed for lesions and necrosis. Remember, PMTV is more often spread as a hitchhiker in powdery scab spore balls on the surface of the tuber rather than within the tuber. At this time, most seed certification agencies do not have tests or procedures in place to regulate PMTV or powdery scab. The one exception is Colorado State, which currently offers a soil bioassay for powdery scab; however, it is only available to growers in Colorado. New tests for powdery scab are being validated at NDSU and are currently available in a beta-test format. For more information see Diagnostic Testing and Questions to Ask Before Purchasing Certified Seed Potatoes.
  • Do not allow contaminated equipment onto your farm. Clean and sanitize all machinery and vehicles that may have come into contact with infested soils or scabby tubers before it comes onto your farm. Don’t forget to include equipment belonging to temporary help. Follow the same procedures when borrowing or leasing used equipment.
  • Avoid manure and compost that may contain spore balls. Ensure that the materials you use from outside sources do not originate from contaminated materials, such as potato debris and culls. See reference below.**
  • Do not plant other potentially infected crop hosts such as sugar beet, oat and tomato.
  • Test potting mix for powdery scab before using it for minituber production and before planting minitubers on your farm. Send samples to a diagnostic lab for testing.6 It is not possible to test for PMTV in the potting mix at this time.
Manage inoculum levels on your farm. Avoid spreading PMTV and powdery scab from infested fields to clean fields.
  • Monitor powdery scab levels in your fields. Testing for powdery scab spore balls helps you to make informed decisions about your fields. For more information see Diagnostic Lab Testing.
  • Practice strict on-farm sanitation protocols. Avoid moving contaminated soil, plant debris or manure to clean fields.
    – Clean and sanitize all machinery and vehicles that have come into contact with infested soils or scabby tubers on your farm. Don’t forget to include equipment belonging to temporary help, contractors and anyone else entering the infested field. Follow the same procedures when borrowing or leasing used equipment.
    – Do not return culls or potato debris to the field, destroy this material instead.
    – Contain water and soil run off/waste generated from washing tubers to avoid contaminating other fields. Do not irrigate clean fields with water that contains runoff from infested fields.
    – Do not reuse bags or containers that have been used for potato transport unless they are clean and free of soil.
    – Restrict movement of soil from infested fields with hedgerows or sod barriers between fields. Do not use headlands, farm or public roads as turning areas.
    – For more information please see Cleaning and Disinfecting Potato Equipment and Storage Facilities.
  • Avoid and destroy alternative hosts. If you need to use an infested field for other crops, do not grow or rotate with another host crop, such as tomato, sugar beet and oat. Destroy weed hosts, such as nightshade and lambs quarters.
  • Manage soil moisture especially during tuber initiation and early bulking phases. Irrigate consistently to prevent wet/dry cycles. When possible, reduce hill moisture by avoiding broad, rounded hills. Reduce soil compaction by periodic deep cultivation. Avoid over-cultivating in order to decrease build-up of fine tilth that won’t drain quickly. See reference below.**
  • Manage powdery scab with fluazinam as part of an integrated management plan. Low to moderate levels of powdery scab can be suppressed with applications of fluazinam fungicide (Omega™) applied in-furrow at planting. For details see the Colorado State webinar Practical Management of Powdery Scab (subscription to APS Plant Management Network required). The effect this treatment has on PMTV is currently unknown.
  • Use sorghum-sudan grass as green manure. If using green manure between potato crops, consider a sorghum-sudan grass mix with plow down prior to fall/winter. This practice has been shown to reduce spore levels in Colorado. See reference below.**
Plant potatoes strategically to reduce risk and the spread of disease.
  • DO NOT plant seed potatoes in infested fields. This will spread the disease to other farms and put future sales at risk.
  • Avoid planting ware potatoes in infested fields, if possible.  Use of infested fields will increase the likelihood of spreading the disease to clean fields. If you find a field highly infested with PMTV and powdery scab, do not plant potatoes or other hosts of powdery scab and PMTV. If you absolutely need to grow crops in an infested field, implement strict on-farm sanitation protocols so that the disease organisms remain contained. Crop rotation is not a viable option because powdery scab resting spores, and thus PMTV, can persist in the soil for up to 18 years.
  • DO NOT plant ware potatoes or any other crop in highly infested fields. 
  • Be aware that planting any crop in PMTV infested fields will increase your risk of spreading the pathogen on your farm. If you must grow crops in infested fields, be sure to practice a strict on-farm sanitation protocol.
Use cultivars insensitive to PMTV-induced tuber necrosis and resistant to powdery scab strategically to reduce loss.
  • Grow cultivars insensitive to PMTV-induced tuber necrosis and resistant to powdery scab in infested fields. There are few commercially available cultivars that are completely insensitive to PMTV. Table 1, below, summarize what is currently known regarding the relative sensitivity of potato cultivars to PMTV-induced tuber necrosis. (Gudmestad et al, 2018). In addition, potato cultivars with moderate to high levels of resistance to powdery scab infection of roots and tubers are available as noted in Table 2 (Bittara, et al 2016). Grow the most resistant cultivars in fields with high spore load and only grow susceptible cultivars in the cleanest fields.
  • As noted under #3 above, be aware that planting any crop in PMTV infested fields will increase your risk on spreading the pathogen on your farm. If you must grow crops in infested fields, be sure to practice a strict on-farm sanitation protocol.

Table 1. Potato cultivar sensitivity to PMTV-induced tuber necrosis (Yellareddygari, Whitworth and Gudmestad. 2018. Plant Dis. 102 (6): 1148-1153)

Ratings:

  • Insensitive (overall incidence <5%)
  • Moderately insensitive (overall incidence >5% to 10%)
  • Moderately sensitive (overall incidence >10% to 15%)
  • Sensitive (overall incidence >15%)
  • ……Not tested/planted

Table 2. Potato cultivar ratings based on their susceptibility to powdery scab on tubers and root gall formation. Cultivars differ in their susceptibility to powdery scab root gall vs tuber infection. It’s best to choose varieties that are resistant to both root galls and tubers lesions. In general, smooth-skinned cultivars are more susceptible than russet-skinned cultivars. Research is underway to develop cultivars resistant to both root galls and tuber lesions. (Bittara, Thompson, Gudmestad and Secor. 2016. Am. J. Potato Res. 93 (5): 497-508)

** Davidson, Robert “Practical Management of Powdery Scab” APS Crop Protection and Management Collection webinar, February 2017. Please note that this seminar requires a subscription. https://www.plantmanagementnetwork.org/edcenter/seminars/potato/PracticalMgtPowderyScab/

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