Potato Crop Update - #10, August 18, 2003
	Contents: Potato Growth and Development Update. A.J. Bussan, Vegetable Specialist, UW-Madison, Horticulture PAA Update. A.J. Bussan, Vegetable Specialist, UW-Madison, Horticulture Potato Disease Update. W. R. Stevenson, Department of Plant Pathology, UW-Madison
Potato Crop Growth and Development Update – Alvin J. Bussan, Potato and Vegetable Production Systems Specialist, UW-Madison, Horticulture Department, 608-262-3519 or ajbussan@wisc.edu
The last week brought some of the hottest growing conditions of the season. High temperatures over the weekend reached into the 90’s and were accompanied by high relative humidity. These types of weather conditions likely led to temporary wilting and ultimately stress in the potato crop, especially if soil moisture conditions were low. Fortunately, low temperatures were still in the 60’s over the same time period. Good irrigation management and a sturdy crop will hopefully mitigate any negative impacts on tuber bulking. Potato harvest is progressing rapidly. Many fields are showing the signs of vine desiccation in preparation for harvest. Improvements in markets have increased the demand for WI potatoes thereby triggering an increased pace in harvest. Superiors appear to be yielding well with excellent and consistent size and shape. Red and fresh market russet potatoes also have good size and shape. Yields have been on the high side of average. Digging potatoes with high temperatures over the past few days will result in high pulp temperatures and cooling potatoes is critical to maintaining a minimal shelf life. In addition, potatoes may be vulnerable to infection of lenticels during the packaging and washing process during this time of year. Tuber bulking has been on a far better pace this year relative to last. We recently summarized bulking data through July 29 at Hancock. Potato lines were planted in 36” rows spaced 16” apart. Each plant continues to average 13 to 14 tubers per plant depending on line with and average size per tuber of about 4 oz. That equates to 3 to 4 lb of potato per hill with yields ranging from 355 to 446 bags per acre. Good bulking conditions from the end of July until mid August likely increased size and yield, but we have not summarized the most recent samples at this time. Reportedly, the current potato crop has high specific gravity and high size. Some growers are concerned about the crop getting too much size with a number of tubers being found that are greater than 16 oz in size. This year’s crop is drawing comparisons to the 2000 potato crop. This crop may have better size than the 2000 crop. A potato crop with high specific gravity and large size is going to be more susceptible to bruising. Recall, that percent bruise free was down considerably in 2000 relative to other years which offset the positive aspects of what was otherwise an excellent crop. General approaches to manage bruising in potato include: 1) calibration of harvest and handling equipment to ensure proper care of tubers (minimize impacts and rough handling), 2) harvesting potato with minimum pulp temperatures of 50 F, 3) waiting for 3 wk after vine-kill to harvest potato, and 4) maintain optimal soil moisture after vine-kill and prior to harvest (low soil moisture can promote bruise, too much moisture can promote tuber disease. I will put together a more comprehensive guide to prevent bruising for future newsletters. However, growers should be aware of the potential for bruising and be prepared to take steps to prevent bruising now. Concerns over tubers with too much size are not always an issue for WI potato producers. Growers should monitor the size profile by digging hills in multiple locations within the field. Once potatoes reach the targeted size profile, growers should vine desiccate to prevent further bulking and trigger maturation of the tubers. Earlier vine-killing will reduce bulking and likely reduce yields, but size profiles that are too large for targeted markets may be a costly quality factor. Potato quality factors other than specific gravity and size appear good to excellent in the current crop as well. To this point, there has been little hollow heart or other internal defects. Maintaining even water availability during the recent heat wave is critical to preventing further/any hollow heart. Pink eye has been minimal and few tubers appear affected by other tuber diseases. Recent ET’s have averaged between 0.20 and 0.25” of water requiring near maximum irrigation applications to maintain adequate water availability. Irrigation management is crucial to maintain the high quality of the current crop during the recent heat stress. At this point, there will likely be little yield impact, but quality factors such as hollow heart, tuber shape, and disease may be high for crops that will not be vine-killed until September.
PAA Update - Alvin J. Bussan, Potato and Vegetable Production Specialist, University of Wisconsin-Madison, Horticulture Department, 608-262-3519 or 608-225-6842 or ajbussan@wisc.edu
I recently attended the Potato Association of America meetings in Spokane, WA. It was my first ever PAA meeting and I met a number of my colleagues and industry associates from across the country while in WA. A number of researchers are doing some outstanding work relative to crop and pest management, managing seed age, storage management, potato genetics, and potato breeding. There was an in depth discussion on tuber dormancy, sprouting, and initiation during the first day of the conference. The closer researchers come to understanding the mechanisms controlling tuber dormancy, the better we will be able to devise strategies for managing tuber dormancy and tuber set. Those interested in viewing abstracts can look at this years meeting agenda on the PAA web site. The potato growing region in the west has experienced an unseasonably hot summer with many daily high temperatures reaching into the 100’s. Most researchers and growers at the conference felt the crop came through the potential heat stress in fair shape. Early harvested potatoes were reported to be yielding well. However, long season potato may realize some yield reductions. The processors have finished cleaning up the remnants of the 2002 crop and have begun to process early maturing 2003 potato varieties based on conversations I had with a few growers in attendance. I also had the opportunity to visit a seed grower near Spokane, WA. I paid particular attention to hill shape, hilling equipment, and tillage systems being employed in Western potato production. The systems being utilized are quite different than current WI systems, in particular the use of para-tillers to complete fall tillage to a depth of 24 to 30 inches and the broad use of dammer/diker type hilling equipment. Next year’s PAA meetings will be held in Scottsbluff, NE, during the early part of August. Anyone interested in attending or hearing more about the 2003 meeting please feel free to contact me.
Vegetable Disease Update - W. R. Stevenson, Department of Plant Pathology, UW-Madison, Tel. No. 608-262-6291, Email: wrs@plantpath.wisc.edu
Potatoes Late blight has not yet been identified anywhere in WI (photos). With three to four weeks of the growing season remaining for the majority of our acreage and no late blight as of today, prospects are good that we may not see late blight this year. If this holds true for the remainder of the season, 2003 will be the first year in the last decade with no late blight. I don’t hear too many complaining about that prospect! Dry weather and recent hot temperatures would further retard spread if there any smoldering pockets that have so far not been detected. As we enter the last month of canopy maintenance, growers should continue protective fungicide programs that will also manage early blight. Weather conditions can change rapidly so stay alert to changing conditions. Early blight continues to progress (photos). In some cases, we are observing more early blight than we’ve seen in the past several years. The crop has bulked at a remarkable pace, putting stress on the vines. In some cases, the vines have completed their job and will continue to senesce in the days ahead. Vines that are stressed by a wide range of factors are somewhat more susceptible to early blight infection than non stressed vines. Fungicide sprays that include protectant sprays such as Quadris or Headline early in the season alternated with chlorothalonil, mancozeb or metiram followed by chlorothalonil, mancozeb, metiram alone or mixed with triphenyltin hydroxide continue to provide some of the best early blight control. In our plots, we are also seeing excellent control in plots treated with new chemistries that will be available for commercial use next growing season. Plot Open House – August 29, Hancock Ag Research Station – 10 am to 2 pm Over the past several years, I’ve hosted an open house in our field trials at the Hancock Ag Research Station. This year’s open house is scheduled on Friday, August 29. I’ll be in the plots from 10 am to 3 pm for anyone who wants to stop by and observe how the various fungicide treatments worked on early blight control. I will have data summaries for the season thus far. This gives growers and the industry a chance to observe new chemistries in the field and to visit about disease management practices. See you there! Snap Beans Aphid transmitted viruses are beginning to take a toll on processing beans. The soybean aphid is present on soybeans in high numbers and winged aphids are moving into snap bean fields. In the past three years, we have observed the transmission of cucumber mosaic virus and alfalfa mosaic virus in snap beans. Typical symptoms of virus infection on snap beans include plant stunting, leaf mosaic, dark green leaf blisters, blossom abortion, malformation of pods and discoloration of pods, both internally and externally (Snap bean virus symptom presentation). Over the past two years, we’ve observed varietal differences in cultivar susceptibility. This year, we have 50 plot entries in trials at Manitowoc and W. Madison representing several seed companies. Differences in disease and aphid responses are present in these trials. Information on cultivar susceptibility is critical to the breeders as well as processors as they plan for future acreage since foliar treatment with insecticides seems to provide little relief from virus transmission. Tom German, UW Entomology, is involved in a statewide survey of viruses affecting snap bean acreage as well as the weeds surrounding snap bean fields. Information from this survey will be useful in understanding the sources of virus inoculum and areas of the state where the virus infection of commercial fields is greatest. Cucumbers and other cucurbits Snap beans aren’t the only crops damaged by cucumber mosaic virus. In past years, we’ve seen CMV on cucumbers and other cucurbits. We are particularly interested in CMV on cucumber plantings since most cucumber cultivars are listed as resistant to CMV. In fact the resistance found in current cultivars has been durable for the past 60 years. If growers observe CMV symptoms on commercial acreage (plant stunting, leaf mosaic, blossom abortion and fruit with dark green warts interspersed with light green areas, give me a call. It would help if the grower has the name of the variety being grown and would be even better if they have a small amount of seed left over from the planting. Pumpkins Powdery mildew pressure is picking up in pumpkins (photos). Once mildew activity is observed on the lower leaves, it becomes increasingly difficult to manage powdery mildew with fungicides. Dry hot conditions are most favorable for powdery mildew development so the current conditions are ideal for sporulation and spread. Pumpkin vines are maturing rapidly and hopefully this maturity is accompanied with a good yield. Loss of the canopy can lead to exposure of the fruit to intense sun and loss in fruit quality. Thus it is helpful to maintain a good canopy through Labor Day. There are several pumpkin cultivars with useful levels of resistance to powdery mildew. We have a small trial at the Hancock station this summer where we continue to evaluate cultivar susceptibility to powdery mildew. Plant resistance is a good alternative to weekly treatment with fungicide sprays and something that should be considered by those growers experiencing difficulty in control of powdery mildew.

Current P-Day and Severity Value Accumulations for 2003 (http://www.plantpath.wisc.edu/wivegdis/index.htm)

Location	Calculation Date:	P-Day Total	Severity Value Total
Antigo emerging June 4	8/18	563	48
Antigo emerging June 14	8/18	492	30
Antigo emerging June 24	8/18	421	30
Grand Marsh emerging 5/19	8/18	648	64
Grand Marsh emerging 5/24	8/18	624	64
Grand Marsh emerging 5/28	8/18	602	64
Hancock emerging 5/13	8/18	698	42
Hancock emerging 5/17	8/18	675	42
Hancock emerging 5/25	8/18	628	40
Plover emerging 5/13	8/18	713	28
Plover emerging 5/24	8/18	653	28
Plover emerging 6/3	8/18	591	28