Potato Crop Update - #8, July 30, 2003
	Contents: Potato Growth and Development 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
Summer weather conditions have continued to be mild over much of Wisconsin. Daytime maximum temperature have hovered around 80 degrees with low temperatures dropping into the low 60’s. For most of the growing region, little precipitation has fallen either with the exception of significant rains across northern Wisconsin last weekend. This year's potato crop appears to be responding extremely well to good weather conditions. We harvested bulking samples in research plots at Hancock yesterday, but will not have tuber size estimates for another several days. Most varieties appear to have a reasonable set (10 to 14 tubers per plant) and with continued good growing conditions the crop should have a high size distribution. Many Russet tubers are over 4” long already and should continue to increase in size. Tuber quality appears to be excellent in this year's crop as well. We have seen few tubers with malformed or knobby shape at the present time (except in one study). We have seen little hollow heart in the few tubers I have sliced in half either. We have been searching for pink eye in several studies but have found few tubers with pink eye. To this point, the water profile has been somewhat easy to maintain due to the lack of heat stress conditions or ET’s in excess of 0.25” per day. I wrote briefly about MH30 several weeks ago and several growers have asked if we are approaching the right application time. Applications during this time of year have been shown to improve tuber shape and decrease yield of cull potato. A guideline pointed out on the MH30 label indicates the smallest tubers must be 1.5 to 2” in diameter and elongating at the time of application to prevent reductions in tuber size. Some varieties may need to be at the 2” size to prevent yield reduction. Based on the work at the Hancock Agriculture Research Station in Russet Burbank, MH30 should be applied immediately for crops that will be vine killed in early September. However, crops vine killed in late September should not be treated with MH30 for another 10 days or so. Many growers have been harvesting for the past couple of weeks, but I assume we will start increasing the rate of harvest soon. I have seen a number of desiccated fields and many early maturing varieties have been senescing for a week or so. We will likely harvest our fresh market and red potato trials in 3 to 4 weeks. If you are interested in seeing the tubers or collecting a sample for any reason please contact me. Like I said earlier ET rates have been reasonable for the most part over the past week ranging from 0.17 to 0.25” per day over the past week. Irrigation management should have been rather straightforward unless you caught some of the precipitation in the Northern part of the growing region.
Vegetable Disease Update - W. R. Stevenson, Department of Plant Pathology, UW-Madison, Tel. No. 608-262-6291, Email: wrs@plantpath.wisc.edu
First, thanks to all those who attended last week’s field days at Hancock and Antigo. The attendance was up at both locations, the weather was pleasant and the food was superb! Potatoes There is still no late blight (photos) reported anywhere in the state. Severity values crept upward over the past week for most locations, but we are still running substantially behind last year (this is a good thing). Last year we had collected 136 severity values at Plover vs. 13 this year. In short, weather conditions have not been favorable for the development of late blight. With at least a month to go for most fields, growers should still be using a preventative program to maintain protection should weather conditions change to favor the development and spread of late blight. Early blight (photos) continues to progress in central WI. We are beginning our field surveys to collect early blight lesions and to process the isolates of the early blight pathogen for sensitivity to azoxystrobin fungicide. This is the third year of the survey. Testing of isolates from last year has shown a small shift in isolate sensitivity compared with isolates collected in 1998 prior to azoxystrobin use. At the moment we are not overly concerned with the shift, but the data tell us that the industry has to be very careful in how they manage the use of strobilurin chemistries such as Quadris, Headline and Gem. We continue to advise the use of up to 3 strobilurin sprays beginning at 300 P-Days alternated with a broad spectrum protectant such as chlorothalonil, mancozeb or metiram and then the protectants for the remainder of the season. It also seems prudent to tank mix the protectants with the strobilurin applications as extra insurance against the selection of fungicide tolerant strains of the early blight fungus. Tank mixes of mancozeb or chlorothalonil with TPTH (triphenyltin hydroxide) are also useful protectant mixtures effective on early blight. During the past week, reports of bacterial stem rot continue to trickle in. The cool weather we’ve experienced over the course of the season has contributed to dense plant canopies. The abundant foliage tends to trap moisture and establish a microenvironment favorable to bacterial soft rot. We are very fortunate that late blight is not present because this dense canopy helps to keep the foliage and stems wet for long periods each day and make it difficult to penetrate with fungicide sprays. Treating with copper sprays at this point after bacterial stem rot symptoms have appeared is probably a waste of money, but in later planted fields where the rows are just closing, perhaps the copper sprays may offer some relief. Snap Beans The cool nights with dew coupled with periodic showers and irrigation are all factors favorable for the development of white mold (photos). So far, white mold incidence has been low to non existent. However, growers and field personnel need to be alert to changing conditions and consider fungicide sprays timed with the opening of the blossoms since this is the most susceptible period for plant infection.

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	7/30	411	30
Antigo emerging June 14	7/30	341	12
Antigo emerging June 24	7/30	270	12
Grand Marsh emerging 5/19	7/30	507	41
Grand Marsh emerging 5/24	7/30	484	41
Grand Marsh emerging 5/28	7/30	461	41
Hancock emerging 5/13	7/30	557	25
Hancock emerging 5/17	7/30	534	25
Hancock emerging 5/25	7/30	487	23
Plover emerging 5/13	7/30	559	13
Plover emerging 5/24	7/30	499	13
Plover emerging 6/3	7/30	436	13