Cultural Control Discussion

Potato Late Blight On-line Workshop
February 17, 1996
Late Blight Management
Discussion Forums

Cultural Control

Cultural practices for disease management are generally defined as those preventing the introduction of inoculum (the pathogen or its parts that causes infection) into the field, reducing inoculum survival and build up, restricting build up of airborne inoculum in the field, reducing infection rates, and creating conditions unfavorable for pathogen development. The most effective programs integrate multiple disease management practices and do not rely on a single method for disease control. Several critical research areas pertaining to disease management were identified at the Late Blight workshop in Tucson.

Oospore production is possible when both fungal mating types are present. However, little is known about oospore (sexual spore) management and the role that these resting spores play in disease development. The potential for oospores to overwinter outside living host-plant tissue may necessitate the development of new disease management strategies. Historically, oospores are known to be present in some Mexican production areas. Until recently, potato producers in the United States and Canada had the luxury of not having to consider oospore management.

Also, more information is needed on primary inoculum sources and their relative importance in disease development. Some examples of primary inoculum sources are potato culls, volunteers and seed tubers, tomato and potato transplants in garden centers and home gardens.

Environmental conditions are known to influence late blight development. Soil texture, fertility, hilling, irrigation, drainage, plant density, plant canopy architecture are just some of the factors known to influence late blight development. An evaluation of factors associated with tuber infection and subsequent decay is needed.

Tuber infection can occur in the field before harvest, during harvest, and tuber to tuber spread can occur during storage. Extensive tuber decay by soft-rot bacteria often follows late blight. A better understanding of processes involved with disease development will enable the development of more effective disease management strategies. dditionally, has the emphasis on late blight management increased the risk of loss due to other pests and diseases? For example, management of late blight via fungicide use may result in greener vines at the end of the growing season. Vine growth later in the season may result in problems associated with aphid feeding (potato leaf roll virus and net necrosis), achieving adequate vine-kill, increased tuber skinning and bruising during harvest, as well as increased storage losses due to fusarium and early blight tuber decay.

What is your opinion on cultural practices for late blight management? What needs to be done? Please enter your comments below. For your convenience, a paper on "Potato Late Blight Management through Cultural Practices" is included in the "Potato Late Blight Online Workshop."

Howard F. Schwartz - 10:58am Feb 25, 1997
I am interested to know how widespread are volunteer potatoes in different production systems? Are there any estimates on the incidence, i.e. 1 - 5 volunteer plants/acre; are volunteer potatoes more evident in subsequent crops of small grains, other vegetables, etc.?

In northeastern Colorado, I have frequently found volunteer potatoes within dry bean and onion fields following potatoes, I estimate that there have been more than 5 volunteers per acre.

During 1997, we plan to survey more systematically for volunteer potatoes and to monitor for evidence of overwintering on tubers or eyes by the late blight and/or early blight pathogens.

Howard F. Schwartz Colorado State University

Hamm Phil - 09:31pm Feb 25, 1997
Howard:

Volunteer potatoes pose a large threat as possible sources of primary inoculum in the Columbia Basin of Oregon and Washington. A survey of year old potato fields over several years revealed more plants in these fields as volunteers than current years potato fields. I will be more specific with numbers when I return to my office.

This past season when the late blight severity in the basin was mild, a least one local chemical technical representive looking at volunteers in a corn field found late blight. While likely rare overall, still it could be, as you know, a source of important primary inoculum.

Jean Ristaino - 10:07am Feb 26, 1997
We are beginning to use a PCR method to screen tubers for the pathogen prior to planting. Seed potatoes are a potential source of inoculum in eastern North Carolina potato fields. We intend to survey seed stocks prior to planting this year to determine the frequency of occurrence of the pathogen.

David Anderson - 12:30am Mar 12, 1997
I would think that volunteer occurrence in our fields runs easily 150 to 200 plants per acre even with local Food Bank Volunteers gleaning behind the harvester. One observation; wireworms will do much more damage to Burbank Russet volunteers in a field of Yukon Gold an organic grower could use the 'Burbanks as a sacrificial host. Back to blight in the Fraser Valley,British Columbia blight is a "given" and agressive fungicide application a way of life here. Copper applications start when the potato plants are 4-6" high and in a wet summer will be repeated every 7-10 days with fungicide type rotation during the season, with an "ear to the ground" for the European and hybrid blight risk factor in the area. David Anderson "Spuds N'Stuff" Abbotsford BC

Philip Hamm - 12:38am Mar 14, 1997
David:
I see that you use copper fungicides, apparently alone. Based on data from a number of sources, the use of copper fungicides alone seems to provide less foliar disease control then does copper tanked mixed with a EBDC or Chlorthalonil. When copper products were used alone in 1996 in the Columbia Basin of Oregon to control Late Blight, poor control was obtained. One other use, to prevent late blight infection in tubers in the field, has not been adaquately proven.

Michael David Coffey - 03:43pm Apr 17, 1997
Volunteers have been a major problem with potato late blight in California in the last two years in part due to successive mild winters. From December through March the early crop has been threatened but early onset of fungicide application from emergence in many cases has saved the crop from total loss. We know this because volunteers are 100% blighted within a few weeks of their emergence. In potatoes we are dealing mainly with an A1 strain characterized as g11 using 9 different RAPD primers. This is not identical to a reference isolate sent to us as US-11, though it is related. In fact we see three related genotypes..g11, g44 and g47..in potato fields these last two or three seasons. Of these g11 is by far the most common. This strain g11 has now become established in the tomato crop where previously we mainly had US-7.