WISCONSIN FRESH FRUIT AND VEGETABLE NEWSLETTER

August 15, 2007

Contents

Good Agricultural Practices

Leucostoma Canker

Vegetable Report

Door County Report

Degree Days

Drought Monitor

UW Specialists

Good Agricultural Practices:
Food Safety Begins on the Farm

            When growers see information about Good Agricultural Practices it is usually related to pest management, fertilizer management, or conserving natural resources such as water and soil.  A new set of Good Agricultural Practices are being introduced to cranberry production that are about a different kind of conservation: Conserving our customers and our good name as an industry.

            Seemingly, every few weeks we read reports in the media of outbreaks of illness related to consuming fresh or minimally processed produce.  The fall and winter of 2006 was especially busy with outbreaks of illness from eating spinach, lettuce, mushrooms, and a recall of peanut butter that tests showed might be contaminated.  Unfortunately, these headlines don’t build confidence in our food supply.  Nutritionists encourage us to eat more fresh fruit and vegetables; while at the same time eating more fresh produce has led to illness and even death.  That is a mixed message that is hard to resolve.  Why are we seeing more outbreaks of food borne illnesses related to fresh produce and what can growers do to reduce the risk?

            The increased incidence of food borne illnesses has multiple causes.  Americans have been encouraged to eat more fresh fruits and vegetables and over the past 25 years per capita consumption of fresh produce has increased sharply.  Fresh produce is available year round while 50 years ago most produce was seasonal.  This requires that produce be grown distant to where it is consumed.  More produce is grown in foreign countries and imported to the United States (although the incidence of food borne illness from foreign produce is not higher than for domestic sources). Americans are eating more meals away from home, in both restaurants and institutional settings.  When there is a problem with food in a large kitchen it can affect many people, not just a single family.  Over the past 20 years new pathogens have emerged that are especially virulent.   Changing demographics include more elderly people and people with compromised immune systems who are more susceptible to microbes.  All of these factors contribute to an increased incidence of food borne illness.

            A broad range of microbes (bacteria, parasites, and viruses) can cause food borne illness.  Bacteria like Salmonella, E. coli O157:H7, Shigella, and Bacillus cereus are of significant concern.  Parasites like Cryptosporidium and Cyclospora, and viruses such as hepatitis A and Norwalk have also caused food related illnesses.  E. coli O157:H7 is of particular concern because the infective dose is very small and the illness it causes is so severe.  The infective dose for Salmonella is millions of cells.  The infective dose for E.coli O157:H7 is as few as 10 cells.  Salmonella will give you an upset stomach, perhaps with nausea and diarrhea.  E. coli O157:H7 can cause bloody diarrhea and may cause permanent kidney damage leading to death. 

Food can become contaminated with these disease causing microbes anywhere between ‘field and fork’.  We can’t control everything that happens to our products once they leaves our control, but we can take steps to insure that the product that leaves our loading dock is as safe as possible.  Most of the steps we can take just show common sense.  Others may require some structural changes in the way we do things.

The goal in setting up food safety Good Agricultural Practices (GAPs) is not to eliminate the risk of contamination.  That is impossible.  Our products are grown outside and we can’t completely control the environment in which they grows.  The goal is to reduce the risk of microbial contamination as much as possible.

Spread of food borne illnesses is usually related to unclean water, poor manure handling, poor personal hygiene, and poor practices for handling and shipping.

Water is intimately tied to agricultural production. Water is used for irrigation, for cleaning produce, for cooling, and in processing.  The key to water safety is using water of sufficient quality to its use. Water used for irrigation can be of low quality. If microbes are present in irrigation water many microbes will be killed by exposure to UV light or will not compete among the soil microfauna. Water used in dump tanks should be chlorinated and the chlorine level of that water should be tested regularly to ensure that sufficient active chlorine is always present. Water used for final cleaning of fruit after harvest should be of potable quality. Wells that provide this water should be inspected regularly to ensure that the casings are intact and functional. Wells should be tested at least annually for coliform bacteria. Water used in processing facilities must be of potable quality. Equipment in processing facilities must comply with local food handling codes that ensure cleanability. Equipment must be cleaned and sanitized daily.

Manure is sometimes used as a source of nutrients.  Sufficient time must pass between application of manure to land and planting or harvesting crops.  90 to 120 days are usually considered safe intervals.  However, feces may end up in fields from rodents and other wildlife. Domestic animals, especially dogs, are sometimes in fields and may leave feces behind. Dogs should be trained to stay out of planted fields. Little can be done about bird droppings or feces from wildlife.

The most important means of preventing the spread of food borne illness is for workers to practice good personal hygiene. Workers should be taught good hygiene and these practices should be modeled by supervisors. Workers (both field workers and harvest/handling workers) should wash their hands as they begin work, after going to the toilet, before and after eating and before smoking. Facilities for hand washing should be available on every property. Proper hand washing takes time. Get hands wet and add soap. Rub hands together and scrub for at least 20 seconds. Rinse hands under running water and dry with single use paper towels. 94% of people say they wash their hands after using the toilet, while only 68% of people actually do wash their hands based on observations.

Workers should wear ‘clean’ clothing to work. This means that clothing should be laundered regularly to remove surface soil, etc.

Workers who are sick should not be at work, particularly in sorting or processing facilities or during harvest or packing. Workers who contact produce directly should have any cuts or wounds bandaged and gloves may be required to prevent direct contact with fruit.

During and after harvest containers that will hold produce should be clean and in good repair. All containers should be cleaned prior to harvest. Vehicles used to transport fresh fruit and vegetables should be inspected and cleaned prior to use. If a truck was used to haul grease or meat prior to cranberries the truck should be thoroughly and completely cleaned. Trucks used for fresh fruit should have sufficient refrigeration capacity to keep the fruit cool, both for maintaining product quality as well as to reduce the opportunity for microbe growth.

Food safety is everyone’s job.  If a foodborne illness outbreak originates in Wisconsin it won’t be the single grower who is implicated; but the entire industry.  Do your part to help the entire industry.

Teryl R. Roper, UW-Madison, Extension Horticulturist

Leucostoma Canker

Participants at the 2007 Wisconsin Apple Field Day witnessed a serious canker disease caused by the fungus Leucostoma cincta at the Peninsular Agricultural Research Station (PARS). Leucostoma canker was first identified at PARS about 10 years ago, when it devastated two separate blocks of Marshall McIntosh. After about two years of losing scaffold limbs and entire trees, the affected blocks were removed altogether. The disease then went into hiding at PARS until this year when it resurfaced in a block of Marshall McIntosh and Red Delicious. I want to take this opportunity to put into writing some of the comments made at the field day.

Leucosotoma cincta is a common canker pathogen of peach trees, but only rarely has the fungus been associated with apple cankers in Europe, Michigan, and a few other sites in Wisconsin. During the outbreak at PARS 10 years ago my graduate student Diane Brown determined that the strains of Leucostoma cincta isolated from apple at PARS were genetically distinct from isolates that originated from stone fruits. We weren’t able to find strains from Europe or Michigan for comparison. Thus, strains of the fungus may be specialized for a particular host, but we couldn’t make a firm conclusion with so few fungal isolates to study. We also are not sure where this pathogen comes from. Presumably it resides on either wild Prunus or Malus trees and then once established in an orchard, it can linger for years.

Because Leucostoma canker is relatively rare, it has not been studied much and there is a lot we do not know. But, there are clues to distinguish this canker from other canker problems in the orchard. Leucostoma canker is an aggressive disease. PARS staff saw no signs of it last year, but by late July of this year it had destroyed several vigorous trees and several scaffold limbs on other trees. This is quite different from cankers caused by species of Botryosphaeria (black rot and white rot). Black rot and white cankers typically develop slowly over a few years on older trees that lack vigor or on limbs weakened by fire blight. Black rot and white rot take out a branch here and there, but these diseases can usually be managed by pruning out the dead wood and making sure trees receive adequate moisture. By contrast Leucostoma canker is fast. The fungus apparently infects trees in fall and/or spring through pruning wounds or winter cracks and then grows rapidly in the spring. By summer, its growth slows down, but the damage become apparent when leaves suddenly wilt and branches die back. Unlike fire blight, the cankers are not moist. Instead they reddish-orange, dry and develop fungal fruiting bodies in the year that foliar dieback occurs. Fire blight cankers may become infected by fungi and eventually become covered with fruiting bodies, but this generally does not happen in the first year. If fruiting bodies are present, the type of canker can usually be identified by sending in samples to the Plant Disease Diagnostic Clinic, 1630 Linden Drive, Madison, WI 53706.

As with other diseases, managing Leucostoma canker entails pruning out diseased tissue and then destroying it or removing it from the site. Drought stress exacerbates canker diseases, so irrigation in dry years is a must. Canker fungi get all the moisture they need from tree bark. Therefore, you must water the trees to give them a chance to fight back! Little is known about the ability of fungicides to protect trees from Leucostoma. Since infection happens in the spring, fungicides used for scab control might help prevent infection by Leucostoma. Once the fungus is inside the tree, however, it is out of reach of fungicides. Most fungicides are not labeled for fall application, so this would not be an option to prevent infection in the fall. The relative susceptibility of cultivars is not known. At PARS, Marshall McIntosh has been worst affected. But many other cultivars have been affected at other sites in Wisconsin and in Michigan.

Fortunately, Leucostoma canker is relatively rare. Because it is an aggressive disease, however, growers should be aware of it.

Patty McManus, UW-Madison, Extension Fruit Pathologist

Vegetable Report

Tomato fruit color

Now that tomato fruit have begun to ripen, growers are beginning to notice various anomalies in the uniformity of the fruit color.  Specific observations include the appearance of blotchy ripening, tomato yellow shoulder, and grey wall.  Each of these disorders is characterized by uneven color through the tomato fruit.  In each case, the disorder is believed to be a physiological disorder that persists even after the fruit has ripened.  More importantly, each disorder causes the fruit to develop a woody and sometimes bitter flavor reducing the salability of the crop.

Although the specific cause of blotchy ripening, tomato yellow shoulder, or grey wall is unknown, several factors are believed to contribute to their development.  Stressful growing conditions such as drought, warm temperatures, or sudden changes in the environment can certainly contribute to the development of all three disorders.  However, adequate potassium fertility has been shown to contribute to the development of blotchy ripening, tomato yellow shoulder, and grey wall. 

If any one of these disorders is developing and persisting within your tomato crop, evaluate you potassium fertility program closely.  Potassium is important for fruit color development in tomato.  Tomato requires 180 lb/a K₂O which is much more than is required for other vegetable crops such as sweet corn, pepper, or pumpkin.  Potassium deficiencies have occurred in multiple problem fields I have inspected with tomato yellow shoulder or grey wall.  Within several of these fields manure was applied to meet the nitrogen demand of tomato, but the application resulted in 1/3 to ½ the required K rate.  In one other case, no potassium was applied, even though nitrogen and phosphorous had been applied.  Potassium fertility needs in tomato can be met with broadcast applications of potash in the fall or spring prior to planting.  If disorders are already occurring, foliar fertilizers containing potassium at appropriate rates can be applied to help alleviate the disorder.

A.J. Bussan, Department of Horticulture, UW-Madison

Early maturing pumpkins

Powdery mildew, drought stress, and/or inadequate fertility have led to early vine death and resulting maturation of pumpkin and winter squash in several areas of Wisconsin.  The question is what to do with the ripened fruit 5 to 6 weeks prior to the targeted harvest date.  I would recommend harvesting the crop and storing fruit in well ventilated building.  Cut the fruit off of the vines with a sharp shears.  Do not set the fruit directly on the ground or on cement, rather place on racks several feet off of the ground.  Keep the fruit out of direct sunlight to minimize potential for sun damage on ripened crop.  Fruit setting in direct sunlight can quickly warm to greater than 95 F which can cause damage to the tissue. 

Pumpkins and winter squash can typically be stored for 2 to 4 months.  Store whole mature pumpkins in a dry, airy location.  Cure pumpkins at 80 to 85 F with 80 to 85% relative humidity for 10 to 14 days after harvest.  This allows the fruit to heal any damaged tissues that occurred during handling. Preventing any damage to pumpkins during harvest minimizes potential infection points for pathogens into the fruit.  After curing, store the pumpkins and squash at 50 to 55°F with a relative humidity of 60 to 75%. Remove pumpkins showing any signs of spoilage from storage shelves quickly. 

A.J. Bussan, Department of Horticulture, UW-Madison

Vegetable Insect Report

Adult moths of the European corn borer (ECB) resulting from the second generation continue to be prevalent across south-central and west central areas of the state (http://pestbulletin.wi.gov/) where the 1,550 GDD threshold has been surpassed. The current degree day accumulations further indicate that the peak period of flights of summer moths has been reached in these areas (in a line extending from La Crosse southeast to Beloit). In turn, we are entering into a time when peak oviposition should be occurring. Sweet corn, snap bean and pepper growers should continue to closely monitor the black light trap results for their particular region or locality and closely monitor fields for eggs and early instar larvae.

Statewide survey results aimed at monitoring developing populations of soybean aphid suggest that only localized areas in the western and west central portions of the state have exceeded economic densities warranting control in soybean. Last week, however, a very significant flight of alatae (winged) soybean aphids was observed across much of south and west central portions of the state. This large flight occurred despite soybean aphid infestations that have been considered below action threshold(s) in many other areas of the state averaging fewer than 100 apterous (wingless) aphids / plant. For snap bean growers, this flight could signal an elevated risk for significant transmission of non-persistently transmitted plant viruses including alfalfa mosaic virus (AMV) and cucumber mosaic virus (CMV). Viruses such as the CMV have infected thousands of acres of snap beans in Wisconsin in past years. Aphids transmit CMV in a non-persistent, stylet-borne manner, which means that viruses are acquired from infected plants and spread to other plants within a matter of seconds. Aphids spread AMV and CMV into snap bean after they acquire it from forage crops like alfalfa and clover plus non-crop weeds. Epidemics of CMV in snap bean have coincided with the recent introduction of the soybean aphid, a now documented vector of CMV. Snap beans infected at an early developmental stage with CMV may yield few to no pods or produce pods that are small, twisted or necrotic. Applications of insecticides to kill aphids before they spread these viruses to other snap bean plants do not work effectively. Management of viruses by using reflective mulches to repel the aphid vector, row covers to exclude aphids and mineral oils to interfere with aphid probing are often not considered an economically viable option in our snap bean crop.

Russ Groves, Vegetable Crop Entomologist, UW-Madison

Other Vegetable Crops: Cucumbers, Melons, Pumpkins and Other Cucurbits –

Disease Alert – I’m still quite concerned about the possibility of downy mildew appearing in the weeks ahead. I still don’t have reports of downy mildew in Wisconsin, but with the disease reported in both Illinois and Michigan, it’s probably just a matter of time before we begin to see downy mildew symptoms on cucurbits in Wisconsin. Remember that there is a long list of possible materials to use for downy mildew and that the materials that work well on powdery mildew don’t work well for controlling downy mildew. For management of downy mildew, registered products include Previcur Flex, Forum, Ranman, or Tanos, each mixed with chlorothalonil or mancozeb. Gavel (already contains mancozeb) can also be used, but does not need a tank mixed companion product. For powdery mildew, consider a long list of materials that includes Flint, Pristine, Nova, Topsin-M, Procure, Quadris, Amistar or Cabrio. Please review reentry intervals, PHI’s (preharvest intervals) and worker safety precautions for each material before use.

Aphid Transmitted Virus Diseases: A large number of winged aphids were observed during the past week at West Madison. Growers should be aware that this phenomenon is occurring and that we will likely see an increase in virus-related symptoms on a wide range of crops including cucurbits and snap beans. Past studies at the UW by the Department of Entomology have shown that insecticide sprays applied to these crops don’t generally have much effect in the subsequent level of symptomatic plants. As aphid clouds pass over an area feeding and probing on all things green, they may pass over a field in a matter of hours and move on their way having left behind viruses acquired from the plants probed prior to their arrival in your field. This often occurs quickly and without growers being aware of aphid presence. Be aware that we may see symptoms appearing on susceptible crops during the next two week period. Symptoms include mild to severe mosaic of leaves, leaf cupping, plant stunting, fruit deformation and blossom drop.

Snap Beans – Last week’s rain, hail and wind likely triggered localized outbreaks of bacterial brown spot. For those growers who sprayed with fixed copper materials immediately after the storms, losses should be minimal. This is also a time to compare varieties in commercial fields hit with the storms for their ability to resist plant infection. Breeders are making good progress in breeding for resistance to bacterial blights as well as other diseases such as root rot.

Drought Conditions

UW Fresh Fruit and Vegetable Contacts