Potato Crop Update - #9, August 7, 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 optimal for potato growth and development. Daytime maximum temperatures have continued to hover around 80 degrees with low temperatures dropping into the low 60’s. Late bulking has continued at a good rate with the continuation of favorable weather conditions into early August. Rain showers were isolated and in some cases heavy across potato production regions over much of the state. Some areas received in excess of 3 to 4” of precipitation. Healthy crop canopies and well managed soils should help minimize the impact of heavy rains in these regions. Many fresh market potato growers have been harvesting for several weeks, with some potato beginning to be harvested for processing as chips. Early harvested processing potato may not be harvested for a couple of weeks yet. Quality appears to be good with the developing crop. We have found little hollow heart (3 russeted potato tubers out of 500 sampled for bulking), little tuber malformation, and good tuber solid content. Common scab is apparent on a number of tubers from one trial at the Hancock Agricultural Research Station, but there are few tubers affected by pink eye or other tuber diseases. Bulking samples from last week weighed between 4 and 8 oz for most tubers, with a few exceeding 10 oz. in size. Russet tubers have excellent length to width ratios with smallest tubers being 2” long and some tubers exceeding 4” in length. A90586-11 had the largest size profile, likely due to a lower plant population and tuber set than Russet Burbank or W1348-rus. Potato vines for most late maturing vines appear to be approaching the maturation phase of development in many fields. At this point little early dying appears evident in any of my plots with one exception. Several disease issues such as white mold or bacterial soft rot of vines may be appearing and Walt may discuss this further in his update. Good potato growth conditions have resulted in manageable ET rates. For most potato producing areas daily ET has been hovering around 0.20” per day. As a result, most growers have been able to prevent drought stress across most fields. Recent heavy rains likely required a reduction in irrigation amounts depending on precipitation events across the region.
Vegetable Disease Update - W. R. Stevenson, Department of Plant Pathology, UW-Madison, Tel. No. 608-262-6291, Email: wrs@plantpath.wisc.edu
Potatoes There are still no reports of late blight in the state (photos). If there had been inoculum moving through the area, conditions were ideal over the past week for the development and spread of late blight. Severity values took a big leap over the past week, so if sporangia are present in the area, we should be seeing symptoms soon. On the other side of the equation, this is latest date in the past 10 years when I can say we haven’t yet seen late blight. It would be nice to keep it that way since the crop appears to healthy at this point of the season. Many fields of reds, round whites and seed are being killed by vine desiccation in preparation for harvest. Fungicide inputs for these early fields are the lowest they’ve been in the last 10 years. For the later planted fields and those fields destined for storage, we still have a month to go in the growing season. Much can happen in a month, so it’s important to maintain coverage of the plant canopy with fungicide through vinekill. At this point, the standard protectants should be sufficient. Early blight (photos) pressure in the state is variable as you move from field to field and between production areas. In the Antigo area, you would have to look hard to find even a single lesion of early blight. There is increasing early blight pressure in central WI and the highest pressure appears to be in the Spring Green and Grand Marsh areas. For those fields where early blight is a visible problem, growers should carefully consider their programs of rotation, irrigation and fertilization. Plants deficient in nitrogen are more susceptible to early blight than well nourished plants. We are in the midst of collecting early blight lesions from the key production areas in the state for evaluation of isolate sensitivity to azoxystrobin fungicide. Results of this survey and surveys in previous years are helping to formulate an effective resistance management program for use of strobilurin fungicides. The month ahead is a critical period in terms of tuber quality and pathogens that could potentially attack the tubers. Over irrigation can play a role in diseases such as pink rot and bacterial soft rot. Keeping mature tubers excessively wet leads to a swelling of the tuber lenticels, thereby creating natural wounds through which zoospores of the pink rot and late blight pathogens along with soft rotting bacteria can potentially enter the tubers. Over irrigation can also erode the soil from the sides of the hills, leading to exposure of tubers to sunlight and elevated temperatures. Exposed tubers are also placed at risk to infection by fungal and bacterial pathogens. Careful and timely irrigation to match crop needs is critical to the production of healthy tubers and long term storage. Snap Beans During the past week we’ve noticed intense aphid pressure on snap bean plantings in southern WI and the pressure is building on plantings in central WI. While the soybean aphid doesn’t normally colonize snap beans the way they colonize soybeans, the effect of high aphid pressure on young snap beans can be profound. At our W. Madison field trial where we have 50 cultivars and breeding lines in a trial to evaluate virus susceptibility, plants which should be in their second trifoliate are exhibiting severe plant stunting and leaf rolling. I’ve never seen this type of injury before and after treatment with insecticide to reduce aphid pressure, we are hoping that the plants can recover to the point where we can then evaluate the plants for virus susceptibility. A second planting just emerging is also exhibiting severe leaf curl. The winged aphids likely already transmitted the stylet-borne viruses we are interested in to our snap bean plantings, but it would be nice to produce at least some healthy-appearing foliage on these plants before the development of virus symptoms. It will be extremely interesting to see the impact of both aphid and virus pressure on yield. Some processors suffered severe losses to viruses in the 2000 and 2001 cropping seasons. In past years, insecticide treatment for control of virus transmission to snap beans has produced questionable results. Hopefully growers treating soybeans for aphid control will see reductions in the level of virus symptoms on nearby snap bean fields. In the end, plant resistance to aphids and virus-incited diseases is the most sensible approach. We are seeing differences in plant response to aphid pressure this year and during the past two years, we’ve seen significant differences in virus susceptibility among cultivars and breeding lines. The two W. Madison and one Manitowoc trials should give us some valuable answers this summer. Beware of white mold on stems and pods over the next month (photos). Cool nights, warm days and long periods of soil and plant wetness during bloom are key ingredients in the development of white mold. We’ve already seen white mold on potatoes this season and I anticipate development of white mold on snap beans during the weeks ahead. Warm temperature above 85F is unfavorable to the fungal pathogen causing white mold and extended periods of warm weather during the bloom period can make a big difference in the amount of white mold that develops. Fungicide sprays at bloom are helpful to control as is crop rotation with non susceptible crops and the use of biological materials such as Intercept applied at or prior to planting the snap bean crop. Carrots Alternaria and Cercospora leaf blights are increasing in distribution and severity in some fields with a history of carrot production. Spread appears to be most rapid on disease susceptible cultivars in fields with overhead irrigation where there are prolonged periods of leaf wetness. Fields should be scouted regularly for disease symptoms and protective fungicides applied accordingly. Melons There’ve been a few questions about the spotting of melon and pumpkin leaves. As a rule, Angular leaf spot lesions have sharp angled corners when they are bordered by leaf veins. The lesions are often watersoaked on the undersides of the leaves. Anthracnose lesions are typically circular and about 1/4 to 1/2 inch in diameter. On fruit, anthracnose lesions are slightly depressed and under moist conditions, the fruit lesions are often covered with an orange mass of spores. The fungal pathogen spreads rapidly by irrigation and rainfall and can be a killer disease. Alternaria lesions have the typical target appearing lesions with alternating dark brown and light tan rings. The lesions can be angular when bordered by leaf veins and are often up to 1/2 inch in diameter. Alternaria normally develops on older leaves and on plants with insuffient nitrogen. There is a long list of fungicides for control of anthracnose and Alternaria leaf blight. Fixed copper sprays sometimes offer a reasonable level of control for angular leaf spot, but this disease is very difficult to control under overhead irrigation and frequent rainfall.

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	8/06	472	42
Antigo emerging June 14	8/06	401	24
Antigo emerging June 24	8/06	330	24
Grand Marsh emerging 5/19	8/06	562	53
Grand Marsh emerging 5/24	8/06	538	53
Grand Marsh emerging 5/28	8/06	516	53
Hancock emerging 5/13	8/06	612	33
Hancock emerging 5/17	8/06	588	33
Hancock emerging 5/25	8/06	542	31
Plover emerging 5/13	8/06	619	23
Plover emerging 5/24	8/06	559	23
Plover emerging 6/3	8/06	497	23