| Potato Crop Update - #6, July 14, 2003 | |||||||||||||||||||||||||||||||||||||||||||||||||||
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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 |
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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 |
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The potato crop looks great and a number of growers took advantage of the somewhat early crop by harvesting fresh market potato in different areas of the Central Sands last week. The recent cooling trend was much needed to maintain good bulking conditions for the full season potato crops in the Central Sands part of the state. Accompanying the drier weather was increased humidity and wetter conditions which may affect disease pressure on the crop.
We were in the Antigo area last Thursday and many fields have approached row closure. Many growers are irrigating fields in the Antigo area as little to no precipitation had fallen for 7 to 10 days. Many fields were in bloom giving the fields a beautiful appearance. The only potato hills I dug were late planted plots at the farm and tubers were in the early bulking stage and 0.5 to 1” in diameter.
Potato in the Central Sands area also looked with full canopies lodging over the course of the past several weeks. Russet Norkotah, Red Norland and Dark Red Norland are beginning to mature as the vines have begun to dry down. Red Norland and Dark Red Norland are 2” in diameter on average. Atlantic potato are also approaching an average diameter of 2” with Russet potato approaching 3 to 4” average length and largest tubers 4+” in size.
A number of growers have been discussing the use of MH30 and asking about optimal application time. MH30 has long been used to inhibit sprouting and has also been shown to improve the shape of potato when applied during late bulking. Weis et al. determined the optimal foliar application timing for improving shape on Russet Burbank was late July to early August. At this timing, yield and percent A yield was optimized while b (not shown) and cull yield was minimized.
Table 1. Russet Burbank response to MH30 at different application times.
There is limited information on varieties other than Russet Burbank. In the study above, MH30 applications in late July or early August preceded vine desiccation by 5 to 7 weeks. For other varieties, time MH30 5 to 6 weeks prior to vine kill to optimize shape and minimize negative yield effects.
Cool wet conditions have diminished the need for intensive irrigation in the sands area. However, growers should still manage irrigation to help recover from hot dry conditions of the previous several weeks and to maintain a positive balance going into the traditionally warmer period of the summer. Potato are currently in the late bulking phase and drought conditions can lead to misshapen tubers, hollow heart, or reduce sugar quality in the harvested crop. |
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Important
dates to remember:
July 15-17 Farm Technology Days, Waupaca Co. July 22 Hancock
Ag Research Station Field Day ( July 23 Antigo
Field Day (
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| Vegetable Disease Update - W. R. Stevenson, Department of Plant Pathology, UW-Madison, Tel. No. 608-262-6291, Email: wrs@plantpath.wisc.edu | |||||||||||||||||||||||||||||||||||||||||||||||||||
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Potatoes A week deeper into the growing season and no late blight statewide. This is beginning to sound redundant, but it beats the alternative by a long shot. Severity values crept up during the past week, especially as you travel from north to south. In the northern regions of Rhinelander and Antigo, growers are working hard to keep up with irrigation. After a wet start to the growing season, it’s been dry for much of June and July. Sprays are being applied on a 7 to 10 day schedule to establish a base residue of fungicide and to eep new growth protected. The crop is growing rapidly in these northern areas and should be at peak bloom in the next two weeks. As you travel south, there has been more rainfall and weather more conducive for disease development. In the Central Sands area, the crop is beginning to lay down and it is not uncommon for foliage to remain moist from irrigation, dew, rainfall well into midday. Symptoms of early blight continue to progress ever so slowly and fungicide programs appear to be keeping this disease in check. Most growers in the central part of the state are spraying with fungicide every 7 days. Some of the earliest planted fields of early maturing cultivars are being harvested green and still others are being killed in the week ahead. Some of these fields have received minimal fungicide treatment, and with no or very low early blight severity, the growers have an advantage this year of reduced inputs. This will be very helpful for those fields being grown for “Healthy Grown” labeling since the contribution of fungicide to the total pesticide toxicity scores will be minimal this year for those early harvested fields. It’s a great example of using disease forecasting models to target fungicide sprays when there is a clear need and backing off on the intensity of fungicide treatment when weather conditions do not favor disease development.
Some have asked in recent days about the 18 severity value and 300 P-Day concepts used in disease forecasting and IPM programming. Computation of 18 severity values relies on maximum and minimum temperatures each day, the duration of relative humidity periods above 90% and the maximum/minimum temperatures during the relative humidity periods above 90%. We start the severity value calculations at emergence – a point where we can see a greenrow and where there is roughly 50% plant emergence. When we reach a total of 18 severity values, we issue a warning. An additional alert is issued when the first symptoms of late blight appear anywhere in the state. This information is published in our newsletters sent to each grower through the courtesy of the WPVGA. When things are moving quickly, this can sometimes mean two newsletters in a single week since we want all growers to be on the same page of awareness and reaction to the late blight problem. In some years, we reach the spray threshold of 18 severity values early in the season. Once we reach 18 severity values, spray recommendations generated by the Wisdom software for late blight can vary from 5 to 7 to 10 day intervals, depending on environmental conditions and the appearance and progress of late blight in the area. You may recall that we surpassed 18 severity values during early June of last year (2002). At times last year, we gained up to four severity values per day. This year (2003) stands in sharp contrast with 2002 since we didn’t reach 18 severity values until the past two weeks for much of Wisconsin. We’ve been accumulating severity values at a very leisurely pace this year, only one or two per day during rainy periods and none for most of the days. We eventually reach 18 severity values each and every year, but what makes a difference in our response and recommendations is how early and how many days does it take to reach this treatment threshold? The other major factor in late blight development beyond weather conditions and the planting of thousands of acres of late blight susceptible cultivars is the presence or absence of late blight inoculum. This year we started the year with pathogen-free seed since most seed production areas of the US did not report late blight in 2002. We also had a hard winter and thus started the year without volunteer potatoes growing in fields where potatoes were grown last year. In addition, Wisconsin growers have been very careful in destroying cull potatoes taken from their warehouses during the shipping season. Culls were spread during mid-winter and allowed to freeze and rot away or were buried in landfills before the new crop emerged. All in all, we started the year with a low risk of seeing late blight early in the season. How far we get into the growing season without seeing late blight remains to be seen, but at least as of this moment, there have been no reports of late blight in Wisconsin. We also worry about early blight and use a P-Day accumulator to generate early blight recommendations. Once we reach 300 P-Days, calculated from emergence on, our spray recommendations take both the P-Day and severity value totals into account to generate 5 day, 7 day or 10 day spray interval recommendations. The interval is variable depending on prevailing weather conditions and the presence of disease in the area. In dry years like 2003 with a low risk of disease, growers have an opportunity to save fungicide sprays. In wet and cool years such as 2002 with a high risk of disease, growers are provided timely spray recommendations that reflect this risk. Hopefully this information is useful to the industry and continues to provide value to individual growers who use the information in managing plant diseases on their farms.
There were two reports of white mold affecting potato vines in the past week. White mold appears as a decay of vines that is generally covered with a white cottony mass of fungal mycelium. Black sclerotia of the white mold fungus are usually embedded in the masses of fungal mycelium and may even appear inside the hollowed stems, especially in late stages of the disease. As the affected stems die and dry, the affected portions of the stem appear bleached and have a papery white appearance. This is the same fungus that attacks a broad range of broadleaf plants including snap beans, soybeans, cucumber, peas and sunflowers. There are several fungicides registered for control of white mold such as Rovral, Omega and Botran. All are protectant materials and these will be of little value in controlling established infections. White mold development is indicative of long periods of soil and foliage wetness. It’s very possible that growers with extensive white mold are keeping the plants too wet for excessively long periods. Use of an irrigation scheduling program that helps to match water application with water needs of the plant will help to minimize white mold while maximizing plant productivity. Given the appearance of white mold this early in the season, I also expect to see bacterial stem rot developing on wet vines in the week ahead. Now that the vines are lying down and staying wet for long periods, soft rotting bacteria are likely to begin rotting stems injured by wind, hail and other storm related factors. Water management is key to management of bacterial stem rot.
Photos: late blight, early blight.
Tomatoes Early blight and Septoria leaf blight (photo) are increasing on tomatoes in southern WI. Both diseases are relatively easy to control with fungicide sprays. It’s important to treat early with protective sprays before the occurrence of extensive infection of the lower leaves. Protectant fungicide sprays such as azoxystrobin, pyraclostrobin, chlorothalonil or mancozeb witll also control fruit anthracnose.
Peppers Bacterial leaf spot was reported on a commercial pepper planting during the past week. Symptoms include brown to black water soaked and greasy appearing lesions on the foliage. Extensive leaf loss can occur under ideal conditions for disease development and later in the season, the disease can lead to blossom loss and unattractive fruit lesions. The pathogen is seed borne and can also arrive on infected transplants. Treatment with copper sprays (Kocide, Champ) is sometimes helpful, but the main factor is weather. Blowing rains, overhead irrigation and warm conditions favor continued bacterial multiplication and spread. Bacterial leaf blight is one of the most difficult to control diseases on pepper. There has been some progress in breeding cultivars with field tolerance to this disease and growers should consult their seed distributors for details on available planting materials.
Cucumbers and Pumpkins Warm wet weather is favorable for the development of Phytophthora blight on cucumbers that can kill individual seedlings and decay the fruit. Infected fruit become covered with masses of white powdery growth and the affected fruit rapidly decay. Control practices consist of crop rotation to crops out of the cucurbit family, very careful watering to avoid long periods of plant wetting and the use of registered fungicides such as Acrobat combined with fixed coppers such as Kocide or Champ. All currently available cultivars of cucumber and pumpkin appear to be susceptible.
Field Days Ahead: Farm Technology Days, Waupaca Co. - July 15-17 Hancock Field Day – July 22 beginning at 9 am Antigo Field Day – July 23 beginning at 1 pm at the airport See you there! Specimens for diagnosis are always welcome. |
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Current P-Day and Severity Value Accumulations for 2003 (Updated information
at: http://www.plantpath.wisc.edu/wivegdis/index.htm)
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Location |
Calculation Date: |
P-Day Total |
Severity Value Total |
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Antigo emerging June 4 |
7/14 |
292 |
26 |
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Antigo emerging June 14 |
7/14 |
221 |
8 |
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Antigo emerging June 24 |
7/14 |
150 |
8 |
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Grand Marsh emerging 5/19 |
7/09 |
356 |
24 |
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Grand Marsh emerging 5/24 |
7/09 |
332 |
24 |
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Grand Marsh emerging 5/28 |
7/09 |
309 |
24 |
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Hancock emerging 5/13 |
7/14 |
441 |
26 |
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Hancock emerging 5/17 |
7/14 |
418 |
26 |
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Hancock emerging 5/25 |
7/14 |
371 |
24 |
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Plover emerging 5/13 |
7/14 |
431 |
12 |
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Plover emerging 5/24 |
7/14 |
371 |
12 |
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Plover emerging 6/3 |
7/14 |
309 |
12 |
Note: We have experienced difficulty this past weekend in downloading data from the Grand Marsh weather station. Hopefully this is temporary and we will be able to update our website with the newest data in the very near future, possibly before you read this newsletter.