Potato Crop Update - #3, June 17, 2003
	Contents: Crop Growth and Development Update. A.J. Bussan, Vegetable Specialist, UW-Madison, Horticulture Potato Disease Update. W. R. Stevenson, Department of Plant Pathology, UW-Madison
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
Potato growth and development has been quickly advancing with good to excellent growing conditions that have persisted through the course of this spring. Recent warm day time temperatures with low relative humidity and cool night time temperatures have been ideal for potato. Correspondingly, the crop has initiated tuber set and the crop has advanced to the early bulking stage of development throughout much of the sands region. I recently summarized P-Day accumulation through 2003 relative to P-Day accumulation in 2001 and 2002 (Figure 1). To date, we have accumulated 25 to 40 more P-Days in 2003 than 2002 and 10 to 15 fewer P-Days than in 2001. As a result, the crop has developed more rapidly this year than in 2002 with emergence and tuber initiation occurring 10 to 14 days earlier and canopy closure occurring 14 or more days earlier.
Figure 1. P-Day accumulation in 2001, 2002, 2003.
Russet varieties planted prior to April 25 have approached canopy closure (>90%) with the exception of slower emerging and growing lines such as W1348, W1836, and A90586-11. We also have seen some emergence issues with A90586-11 with a 20 to 25% stand loss in our research trials at Hancock. Russet varieties are well into early bulking with tubers ranging in size from ¼” up to 1 ½” and netting has already formed on the periderm. Stem densities of Russet Burbank has ranged from 3 to 4 per hill in surveys of grower seed lot. Round white potato varieties have achieve 80% or more canopy closure with many varieties already flowering. Red varieties have also approached canopy closure (>90%) with Red Norland and Dark Red Norland well into flowering. Round white and red varieties have entered early bulking as well with largest tubers approaching ¾ to 1” in size. Several growers have commented on the relative health of the root system under the current potato crop. In my limited digging, I have noticed well developed roots with healthy looking secondary roots. I will do more extensive digging prior to next week to document rooting depth and expansion between the rows. Early bulking is a critical stage for irrigation management in potato. Research from multiple locations throughout the U.S. has shown drought stress during early bulking can cause the greatest reductions in yield and quality of potato. Cell division is rapidly occurring during early bulking and inhibiting cell division will ultimately limit the potential size of a given potato tuber. In addition, drought stress during early bulking can cause severe sugar end problems which leads to discoloration of fries on the stem end. Most growers have already entered into an aggressive irrigation management plan and potential ET is equal to actual ET when canopy closure exceeds 80%. The past several days have seen high temperatures in the low 80's with night time lows in the 50's.  ET rates have exceeded 0.20"/day for most of the sands.  Growers need to monitor estimated ET closely and implement irrigation management techniques to ensure good soil moisture content.  Drought stress at this point of the season can have dramatic yield effects and reduce tuber quality.
Potato Disease Update - W. R. Stevenson, Department of Plant Pathology, UW-Madison, Tel. No. 608-262-6291, Email: wrs@plantpath.wisc.edu
A week later into the growing season and I remain optimistic for an excellent crop. Stands in most fields are excellent with very little seedpiece decay, blackleg or the usual assortment of early season diseases. Growth is uniform and the plants are growing rapidly. Rows are beginning to touch in many fields and some growers are reporting tubers larger than golf balls. I don’t have much to report in terms of disease. I’ve seen no reports of late blight in the Midwest as of today. There are no submissions to the clinic for diagnosis of late blight. The number of severity values calculated from in-field weather data remain low for central and southern WI. For the Antigo area, however, we reached 18 severity values last Friday. The severity values have not increased since then and weather conditions are expected to be unfavorable for late blight development for the next several days. Many growers have initiated their fungicide spray programs to at least establish spray coverage on the lowest leaves and stems before the rows close. For many Antigo growers, the plants have only emerged during the past two weeks and they are a long ways from row closure. Still, it would be wise for the growers in the Antigo area to initiate fungicide treatment, especially if they can band fungicide over the rows during hilling, a less expensive alternative to broadcast treatments at this time of the year. During the past week, I’ve received a few calls from growers and consultants regarding the accumulation of severity values. Another UW website not maintained by me or my staff uses an alternative source of weather data whereby a computer program predicts in-canopy temperature and relative humidity based on data from satellite imagery and local airports. Using these data for calculation of severity values and a May 15 emergence date, this UW website indicates that there are currently 26 severity values at Antigo, 44 severity values at Hancock, 52 severity values at Plover and 46 severity values at Grand Marsh. At this rate of severity value accumulation, growers would be treating with fungicide every 5 days and we would expect to see late blight very soon, if not in the past week. Let me emphasize that we have not seen any late blight in the state and using data from our in-canopy sensors, the number of severity values at all recording locations (see below) is far lower than the values listed above. We are confident that we are correctly monitoring environmental conditions within the plant canopy and will continue to provide severity value and P-Day calculations on our website for the remainder of the season. I know that this is confusing to all parties and we are attempting to rectify the situation. Please bear with us. This afternoon (6/17) the first sample of early blight was observed at the Hancock Ag Research Station. Since we are currently at 242 P-Days and counting, this observation is about a week ahead of schedule. We plan to initiate our fungicide programs in our test plots at 300 P-Days and this will likely occur on June 25. The first symptoms of early blight appear on the oldest leaves near the base of the plants. Lesions are less than ¼ inch in diameter and are surrounded with a yellow halo. As the lesions enlarge, they generally become angular in appearance when bordered by leaf veins. The surfaces of mature early blight lesions exhibit multiple rings giving the lesions a bullseye appearance. There are many fungicides capable of controlling early blight. The strobilurin fungicides including Quadris, Gem or Headline all provide control of early blight, especially when mixed with a broad based protectant such as chlorothalonil, metiram or mancozeb. Be sure that you do not use back to back strobilurin sprays and I would suggest using only three strobilurin sprays for the season alternating with chlorothalonil, metiram, mancozeb or triphenyltin hydroxide (TPTH) mixed with mancozeb. If Quadris fungicide was used as an in-furrow treatment for control of soilborne diseases, your first fungicide spray should not be a strobilurin material. See my next “Common Tater” article for a more complete discussion of resistance management.

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	6/17	93	18
Grand Marsh emerging 5/19	6/17	192	4
Grand Marsh emerging 5/24	6/17	168	4
Grand Marsh emerging 5/28	6/17	146	4
Hancock emerging 5/13	6/17	242	6
Hancock emerging 5/17	6/17	219	6
Hancock emerging 5/25	6/17	172	4
Plover emerging 5/13	6/17	231	2
Plover emerging 5/24	6/17	171	2
Plover emerging 6/3	6/17	109	2