Potato Crop Update - #11, September 3, 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, cell 608-225-6842 or e-mail or ajbussan@wisc.edu
Potato growth during the latter part of August was influenced by a warm front that resulted in temperatures well above 80 F for several days. However, cool night temperatures and the recent arrival of a cold front have resulted in seasonable conditions. We have been sampling bulking in 3 different russet lines and I am particularly interested in seeing the impact of the heat on the bulking of potato. We dug a number of samples yesterday and the shape of Russet Burbank is starting to take on a pointed appearance on the bud end in some of the plots. Hopefully, this effect will be relatively minor and have little effect on grade. Harvest has started to occur at an increased pace with many fields appearing desiccated across the region. Conditions have improved for harvest with cool night temperatures allowing for management of pulp temperatures. I am hoping for good harvest conditions to continue over the next 6 weeks as I am not scheduled to harvest my last plots at HARS until October 6 to 8. Potato vines have definitely begun to senesce in almost all the crops, but there is a good deal of variability in the current canopy. Early to mid season varieties have little if any green vines left. However, we had serious re-sprouting issues in trials with Dark Red Norland and Red Norland potato. We had 3 to 6” of regrowth on plots that had been vine desiccated twice. I also witnessed resprouting of vines in variety trials adjacent to my plots at the Hancock Agriculture Research Station. Growers may want to scout field several days prior to harvesting to make sure vines are dead or determine if another vine desiccation will be necessary. Long season potato varieties have vines with varying degrees of senescence. In visiting with Walt Stevenson, the crop canopy appears to be in great shape relative to other years. Several of my trials at HARS have little if any green vegetation left in Russet Burbank, whereas trials 50 feet away still have a majority of their canopies remaining. In part, earliest planted Burbank and weedy plots have lost a majority of their crop canopies. Early blight is also quickly leading to vine senescence in my trials with Russet Burbank. We have dug Russet Norkotah, Silverton, Red Norland, Dark Red Norland and several experimental lines at HARS. I haven’t processed numbers at this point but the size distribution appears to be much improved over last year. Specific gravities were near 1.070 or less for all the varieties listed. However, W1201, a round white, had gravities in excess of 1.085 for a number of the plots. Hollow heart was a minor issue with less than 10 % of the tubers being affected. Shape was beautiful on most of the potatoes and should allow for good packaging. Several of the red lines did have a fair amount of scab which could be an issue but had exceptional color. W2275-9R in particular was extremely sharp with a wine red color upon digging. Bruise prevention and management With the potential for high specific gravity in processing lines such as W1201, Russet Burbank, and others, the concern for bruise in potato will be a large issue during the harvest and storage season. Shatter bruise and black spot bruise are of particular concern during the harvest and handling process. A few basic steps to minimize bruise are: -Vine kill 18 to 21 days prior to harvest to ensure good skin set. Good skin set is crucial to minimizing bruise potential. -Maintain soil moisture above 50% of field capacity. The key is to maintain adequate tuber moisture content. The current crop has had good vine and root development so managing soil moisture will be key to minimizing bruise from vine desiccation through to the end of season. Avoid over watering as well to prevent tuber rot or decay. -Do not harvest when pulp temperatures are below 45 F. Do not harvest until the warmer part of the day when night time temperatures are below 50 F. The low temperature at HARS has been below 50 F every day since last Thursday. -Calibrate harvesting and handling equipment to minimize the potential for bruising. Adjust digging chain speed relative to ground speed to keep the conveyors full of potatoes. Make sure digger depth is adequate to lift potatoes. Minimize drops in handling potato from harvester through to the pile in storage. An excellent review of bruise management and prevention can be viewed at the following link: http://info.ag.uidaho.edu/resources/PDFs/BUL0725.pdf (Thornton and Bohl) I strongly encourage everyone to take a few minutes to review this document. If you do not have web access and need a copy contact me and we can make arrangements to get one for you. Spooner field day We had a great tour at the Spooner Agriculture research Station on August 21. Phil Holman has done a great job in managing the plots at the Spooner Agricultural Research Center and organizing the tour. It was good to see everybody at the event and I had a great time during and after the tour. Similar to other areas of WI, the potato crop appears to be in good condition in the northwest. Potato shape looks good and in the few potato tubers we cut in half, little hollow heart or other internal defects were present.
Vegetable Disease Update - W. R. Stevenson, Department of Plant Pathology, UW-Madison, Tel. No. 608-262-6291, Email: wrs@plantpath.wisc.edu
Potatoes The foliar disease situation hasn’t changed much in the past two weeks. There is still no late blight reported anywhere in Wisconsin (photos). There is, however, late blight being reported in some other midwestern states and in eastern states where above normal rainfall continues to fall. It’s simply too dry in Wisconsin for the disease to become established. With the 7-day forecast for no rainfall in the state, the risk for late blight appearance this season continues to be very low. Not having late blight this season is a welcome relief, but it would be desirable to have a timely rain shower. Early blight has been tough to control from mid season until the present week (photos). We’ve observed heavy early blight pressure in our Hancock trials. Some of the treatments containing strobilurin fungicide chemistries provided good control, but not the expected excellent performance levels seen only a few short seasons ago. The isolates being collected from our experimental trials are less sensitive to strobilurin chemistry than isolates collected back in 1998 prior to widespread strobilurin use. Our program is conducting a statewide survey of early blight isolates to determine the distribution of isolates with reduced sensitivity to strobilurin chemistry. Analysis of the isolates takes time, but we hope to have the information ready for the winter educational meetings in January. Other problems of note on potatoes include a high incidence of common scab in a few fields. The problems are mostly confined to Norkotah Russet, but there are some losses observed on red potatoes. Moisture stress at the time of tuberization is likely related to symptom development as is the trend to field liming to raise the pH. Many production fields now test at close to a pH 6.5 or even higher. Keeping the soil pH at a 5.2 to 5.8 pH range coupled with careful water management during early tuberization helps in the management of common scab. Where pockets of scab are detected in production fields, I suggest that growers test the soil pH in individual areas rather than bulk samples from these areas with samples from the rest of the field. We’ve also received the first samples of powdery scab from commercial potato fields in Wisconsin this summer. The first samples to reach us included a root sample with small galls (1/4 inch in diameter) on the roots. The WI DATCP survey crew has now detected powdery scab in additional fields and will initiate a more intensive survey this fall. Powdery scab can be an important soilborne disease leading to open scab blemishes on the periderm and loss of tuber quality. The powdery scab pathogen is also a vector of the potato mop-top virus although this virus has never been detected in WI tubers. You will likely see additional information on powdery scab as the DATCP survey proceeds. Snap Beans Virus woes continue to plague growers and processors statewide (Snap bean virus symptom presentation). Yields are down, but at this point pod discoloration and disfigurement problems seem to be minor compared with previous years. The majority of the pod discoloration complaints were appearing during the hottest weather two weeks ago. With cooler weather now occurring statewide, pod quality complaints are minimal. A statewide survey conducted by Tom German (UW Entomology) is finding predominantly cucumber mosaic virus (CMV) in snap bean and weed samples. It’s interesting to note that in fields in NE Wisconsin visited two weeks ago where plants exhibited severe mosaic and blistering of young leaves, the same plants are now growing rapidly and the newest leaves exhibit only a mild mosaic. There is a reversal of symptom severity in just a two week period and the prospects are good for a respectable yield. This has been a tough year for the soybean aphid, the probable vector of virus pathogens on snap beans, and widespread virus infection of the snap bean crop. It’s interesting to note that there are observable differences in the field reaction of snap bean cultivars and breeding lines exposed to natural inoculation by aphids in our test locations at the West Madison Experiment Station and north of Manitowoc. Anyone wishing to see our West Madison trials should attend the Midwest Food Processor Field Day at the West Madison Station for a 2 pm tour. Pumpkins Powdery mildew is the key disease currently affecting pumpkins and other cucurbits (photos). Affected foliage is covered with a white coating and within a few days, this infected foliage turns brown. Control measures are likely too late if infection has progressed much beyond a few white patches on the upper leaves. The disease spreads rapidly and infected foliage dies rapidly. We continue to evaluate pumpkin cultivars and breeding lines for their field reaction to this pathogen in trials at Hancock. There are several lines with excellent levels of field resistance to powdery mildew. Data from these trials will be posted on our web site as soon as the information can be collated and analyzed.

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	9/01	660	49
Antigo emerging June 14	9/01	589	31
Antigo emerging June 24	9/01	518	31
Grand Marsh emerging 5/19	9/01	745	69
Grand Marsh emerging 5/24	9/01	721	69
Grand Marsh emerging 5/28	9/01	699	69
Hancock emerging 5/13	9/01	798	44
Hancock emerging 5/17	9/01	774	44
Hancock emerging 5/25	9/01	728	42
Plover emerging 5/13	9/01	808	28
Plover emerging 5/24	9/01	748	28
Plover emerging 6/3	9/01	686	28
Lake Mills emerging 6/10	8/31	629	63
Lake Mills emerging 6/12	8/31	612	63
Lake Mills emerging 6/14	8/31	596	61
Lake Mills emerging 6/16	8/31	580	61
Lake Mills emerging 6/21	8/31	548	61