Potato Crop Update - #4, July 1, 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
I haven’t been able to evaluate many potato hills over the course of the last week, but I did want to discuss some of the recent weather patterns. The month of June was dry with most of the Central Sands Region receiving less than 2” of precipitation. Most of the June precipitation fell during the first third of the month with the exception of the storm system that passed over the state last week. The past 10 days have seen a general warming trend with high temperatures reaching well into the 80’s a week ago. ET rates have increased with clear skies, warm temperatures and full crop canopies. The crop looks good with full canopies in almost every field. We have seen vines begin to lodge in Russet Burbank fields making some of our research plots difficult to access in field scale trials. I was concerned about the potential for heat and drought stress, especially from June 20 through June 24. Heat and drought stress during early bulking have the potential for large negative impacts on crop yield and quality (i.e. sugar ends). Potato crops under good irrigation management plans should have come through the month of June in good shape. Reference ET has been hovering between 0.22 and 0.27” at Hancock for much of the last half of June. Minimum irrigation demand has been between 0.50 and 0.55” every other day for much of that time. Much of the potato crop appears to have a healthy root system which should help the crop tolerate high temperatures that exceed 85 F. We are going to start monitoring tuber bulking and tuber set in some of our trials this week. I will have more accurate information on the status of the current crop for next week’s newsletter.
Important dates to remember: July 11 Rhinelander Field Day at Lelah Starks Elite Foundation Seed Potato Farm July 15-17 Farm Technology Days, Waupaca Co. July 22 Hancock Ag Research Station Field Day (9:00 am start) July 23 Antigo Field Day (1:00 pm start)
Potato Disease Update - W. R. Stevenson, Department of Plant Pathology, UW-Madison, Tel. No. 608-262-6291, Email: wrs@plantpath.wisc.edu
The Wisconsin potato crop continues to look good from a disease standpoint. I can’t remember a crop looking this good at this point in the growing season. Most fields are uniform in emergence, canopy height, leaf color and flowering. There are currently no reports of late blight (photo) anywhere in Wisconsin. It’s been awhile since I’ve been able to make that statement on July 1. Rainfall during the past few days helped to extend the periods of high relative humidity. Consequently the number of severity values increased by about 4 SV’s at most monitoring sites. The only area in the state with more than the threshold number (18) of severity values is Antigo. The majority of these severity values accrued in the first 10 days after emergence of the earliest emerging fields. Growers in that area are advised to keep close watch on their crop for symptoms of late blight and to treat accordingly. In the absence of volunteer potatoes and cull potatoes, the likelihood of late blight is small, but it pays to be observant. Once plants begin touching in the row, it would be wise for growers in the Antigo area to begin their fungicide program, even though severity values are now increasing slowly. Most growers in central WI have initiated a fungicide spray program on at least the earliest emerging fields since we are now past the threshold of 300 P-Days where we anticipate an increase in the airborne inoculum of the early blight pathogen. Symptoms of early blight are beginning to appear on the older leaves where fungicides have not been applied (photo). The early lesions can be up to ½” in diameter, angular where bordered by major leaf veins, appear as multiple concentric circles and are bordered with yellow tissue. There are many fungicides with good to excellent efficacy for control of early blight. My recommendation continues to include strobilurin fungicides (Quadris, Headline or Gem) tank mixed with a protectant fungicide such as chlorothalonil, mancozeb or metiram on weeks 1, 3 and 5 after you reach 300 P-Days and treatment with one of the protectant fungicides on the remaining weeks. An additional protectant fungicide mixture that has worked well in past years is triphenyltin hydroxide (TPTH) (Super Tin) tank-mixed with mancozeb or metiram or chlorothalonil. Applying this mixture beginning in late July through early August has provided good early blight control in past years. Remember that you should never mix TPTH fungicide with MH-30 or oil-based insecticides and be cautious about spraying TPTH fungicides during hot humid weather, especially on sensitive varieties such as some of the red skinned varieties. In field visits during the past week, I continue to observe symptoms of Rhizoctonia stem canker on slow emerging and slow growing plants of several cultivars. The typical lesions of Rhizoctonia stem canker include brick-red superficial lesions that girdle affected sprouts and stolons and can kill sprouts before emergence. In other fields I’ve observed a brown to black discoloration and pinching-off of sprouts at a uniform height of about 2-3 inches above the seedpiece prior to emergence. This is not Rhizoctonia stem canker since we are unable to isolate the stem canker pathogen from symptomatic tissues. Symptoms appear to be related to chemical or possibly fertilizer burn since the symptoms appear after hilling and fertilizer application and symptoms from one plant to another appear at a uniform height above the seedpiece. I’ve also observed small amounts <1%) of what appears to be PVY and leafroll in fields for fresh market in central WI (Gold Rush and Silverton). Plants affected by presumed PVY are somewhat shorter than their neighbors and there is a slight mottling and mosaic of affected leaves. Plants affected by leafroll are stunted with severe rolling of the lower leaves. Diagnostic tests are underway. Crops Other Than Potato Soybean aphids are beginning to appear in several areas of the state and there is concern about aphid transmitted virus diseases on snap bean plantings. Virus problems caused severe damage on snap beans in 2000 and 2001 in southeast and eastern WI. With delayed and reduced populations of the soybean aphid in 2002, virus related losses were minimal. Snap bean growers and processor field personnel who observe plant stunting and associated mottling of leaves, blossom drop and pod malformation are asked to give us a call so that we can include affected fields in survey activities this summer conducted by Dr. Tom German (Snap bean virus symptom presentation). We also plan on planting variety evaluation trials at West Madison and Manitowoc during July where we will test approximately 40 cultivars and breeding lines for their in-field reaction to aphid transmitted virus diseases. Recent rainfall and warm conditions favor the development of Septoria leafspot on tomatoes (photo). The fungal pathogen overwinters on leaf debris and spores are splashed onto susceptible foliage by rainfall and overhead irrigation. Leaf lesions are approximately 1/8” in diameter and there can be dozens of lesions per leaflet when conditions are ideal for spore release and leaf infection. The disease moves from the lower infected leaves up the plant until almost complete defoliation occurs. Fungicide sprays including strobilurin materials (Quadris, Cabrio), chlorolothalonil and mancozeb are effective for controlling Septoria leaf blight and will also control other foliar and fruit diseases of tomato.

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	6/30	186	22
Antigo emerging June 14	6/30	115	4
Grand Marsh emerging 5/19	6/30	288	11
Grand Marsh emerging 5/24	6/30	264	11
Grand Marsh emerging 5/28	6/30	242	11
Hancock emerging 5/13	6/30	388	10
Hancock emerging 5/17	6/30	314	10
Hancock emerging 5/25	6/30	268	8
Plover emerging 5/13	6/30	326	7
Plover emerging 5/24	6/30	266	7
Plover emerging 6/3	6/30	203	7