Spring frosts can cause significant losses to fruit crops. Classic radiation frosts with clear skies and calm conditions are common in all fruit growing regions. In a radiation frost, the ground cools by radiation to the sky. The cold ground cools the air above it. It is this cold air that causes frost or freeze damage. It is a good idea to routinely assess the air drainage on your farm. Plant growth and construction will cause air drainage to change over time and maintaining good air drainage can save a grower a crop in years when frost hits a region.

There are numerous sites where a degree or two could be gained by removing obstructions from air drainage passages under radiation freeze conditions.

Air is subject to the laws of thermodynamics and hydraulics. Cold air is denser than warm air. Because of external energy and wind currents the air can have different temperature layers vertically or horizontally. Cold air sinks flowing downhill to the lowest available point where it accumulates until dispersed by heat or wind. Even on still nights there is air movement on slopes. Steeper slopes cause air to move faster. It takes energy to move air through or around obstructions. Therefore, cold air often accumulates above obstacles. For air drainage under calm conditions we are only concerned with low-level air currents from ground level to 200 feet or less.

Mornings with low-lying ground fog (10 to 20 feet) are ideal for assessing air drainage. At dawn go to the highest spot on the farm and make note of where fog patches lie. Walk through these areas noting where fog is thickest and where it thins out. Look for obstructions that keep the fog in the area. Watch for moving fog banks, they show the natural airflow if there is no wind. Follow surface water drainage pattern, it will tell you where cold air travels.

Fruit sites should have large air storage areas within a half-mile or at least allow a general airflow to a storage area within one mile. Cold air storage over bodies of open water is larger than it appears. Heat and water vapor from open water warms the air in contact with it. This causes air to rise, making room for more air to enter.

Even on still nights, air movement down slope creates a slight breeze (one to three miles per hour). One good way to find these breezes is with a helium balloon on a string. The balloon will tip in direction of airflow.

There are basically three things you can do to help air drainage. Remove obstacles to air drainage, maintain drainage ways in good condition and create more cold air storage off site. Solid obstructions block airflow causing it to stop and pile up higher than the obstacle (often 2 to 3 times higher) before it pours over the obstacle. Colder dense air will pile higher than warmer air. Obstructions tight to ground are more effective air dams than obstructions with openings near the ground. Sixty feet is the minimum width opening for good airflow. The larger the area draining through the opening the wider the opening should be.

Maintaining good air drainage is a never-ending task. Analyze every field for potential alternations before replanting. Plant appropriate varieties. Remember that north and east facing slopes get cold slightly faster than south to west facing slopes. Place earlier growing fruits in warmer sites and later fruit in the cooler sites.

Constantly assess air drainage. Any time you have ground fog pay attention to where it is. This usually happens in the fall during harvest as moisture is condensed out of the air. Be aware of air drainage problems off your farm that will affect you. Work with your neighbors to open-air drainage even if you have to cover all expenses. Lastly, don't let the county or state fill a road cut across your air drainage area without compensation.

Spring frosts are a major climatic threat to grapes and other fruit crops in Michigan. Grape varieties that break bud early in the spring are the most vulnerable. The more developed a bud or shoot becomes, the less hardy it is. Cultivar identity determines both the rate of shoot development as well as the actual hardiness at any given stage. Spring frost injury is the most common form of injury suffered by the more winter hardy cultivars.

Although site selection remains the best approach for frost protection, alternatives have included use of heaters, sprinkler systems, and wind machines. These methods are generally expensive and, depending on the type and severity of the freeze, produce varying results. Other methods of mitigating the risk of frost injury have focused on delaying bud break. Delayed pruning or double pruning can be used to delay bud break. Leaving canes or spurs longer than necessary (e.g., by 10 to 15 buds) not only aids in pruning adjustment for winter injury, but also causes buds closest to the old wood to break later. The apical or end buds develop first and suppress the development of the buds nearer the trunk or arm.

The less developed a bud, the hardier it is. After the date of the last probable spring freeze has passed, the canes are shortened to the desired length to properly adjust the bud number for the vine. Growth of the basal buds can be delayed by as much as two weeks if weather conditions are favorable. While this procedure requires an extra trip through the vineyard, it can mean the difference between an acceptable crop and little or no crop. In Illinois, dormant oils (like soybean oil or JMS stylet oil) were evaluated for delaying bud break in Vinifera grapes. Dormant oils are normally used to suffocate insects and mites. The study showed that the application of dormant oils could delay bud break by four to seven days. In Virginia, JMS Stylet Oil application to dormant Seyval vines delayed bud break by 15 days.

Frost control is a major concern of most fruit farmers, but often little is done because of cost. There are numerous sites where a degree or two could be gained by removing obstructions from air drainage passages under radiation freeze conditions. Since cold air is heavier than warm air, it tends to seek the lowest point and flows like water across the farm. Obstructions such as road fills, natural earth dams and dense undergrowth of brush or trees in fencerows slow cold air movement.

By removing obstructions, growers can move cold air more quickly off of the farm and can often reduce frost damage. The use of ground-based fans to improve air movement, and thus frost control, is not new technology and most tests indicate little benefit, especially with the older small units tested, and in locations where large areas were involved.

Several years ago Retired Extension Agent Mike Thomas worked with a grower in Hartford, Michigan, and Phil Brown of Conklin, the manufacturer of a large PTO-driven tractor-mounted fan, in a test of its ability to affect temperature in a very specific situation on the farm. A 25-acre portion of the farm drained through a swale that had a very narrow opening between two hills and a small four-foot high natural earth dam at the mouth of the opening. Another orchard was planted below this opening to the edge of the farm where cold air continued off to lower elevations in a large cold air storage area. The grower had noticed that the orchard beyond the drainage opening had a better crop on it than the orchards uphill from the opening. It was very obvious from the frost line in the orchards that it was colder at several elevations uphill of this obstruction.

The grower widened the opening from about 30 feet to about 100 feet to breach the natural dam in the opening and also cleared brush from the property line below the opening. The situation was ideal for air moving fans because of the unique topographical features. Phil Brown agreed to provide two 100,000-CFM fans for a trial.

A test was run on frosty fall morning. Two fans were used, one just beyond the opening and another about 500 feet uphill in the drainage way. The drainage way is a double curve and the uphill fan was placed to move air around the first curve. The effect of the individual fans was quite different because of the topography. The uphill fan pulled from a wider, flatter area and pushed air into a narrower area. It did not affect temperature near it as greatly as the lower fan at the opening. This fan pulled from a narrow area and pushed into wider, flatter area. It would take a lot of these fans to have a positive effect on a broad slope, but in this specific location the two fans created an increase in air flow which affected temperatures positively over 15 to 20 acres under moderate radiation freeze conditions.

Growers wishing to invest in frost control with ground fans need to look for specific sites on the farm where the fans can be used to speed airflow through natural drainage passages. The more restricted the passage, the greater effect the fan will have. Testing on a frosty night is the only way to be sure it will work for you.

A common question this time of year in the Agricultural Weather Office is, "When can I expect my last freeze this spring?" Thanks to a 50-year data set developed by Dr. Fred Nurnberger, the emeritus state climatologist, the answer is relatively straightforward but depends greatly on your location. Most areas of the state experience last freeze during May, although the records indicate that it is possible for many spots to see freezing temperatures well into June. There is strong dependence on lake proximity with significantly earlier last freezes in areas near the lakes vs. those further inland. Interestingly, the record latest "spring" freeze for the state for this period is at Vanderbilt (Otsego County), on July 30. This station also recorded a freezing minimum temperature on August 2 of the same year (1972). Because of this, the defining limits of the last "spring" freeze and first "fall" freeze in Michigan (this could also be interpreted as a 2-day growing season!) had to be arbitrarily set at July 31st (with the beginning of the fall as August 1st).

The numbers in Figures 1a, 1b, and 1c depict the 10th, 50th (average), and 90th percentile date after which the last freezing temperature (32° F or lower) of the season occurs. For example, in the chart labeled, "90% chance after indicated date in spring," a date of 4/25 or April 25 is given for the Lansing area. This means that in 90 percent or nine out of ten years, the last freeze occurs on or after April 25. Similarly, the ten percent chance date of May 25 for Lansing indicates that in only one out of ten years does the last freeze occur after May 25. In between these "outlier" dates is the 50th percentile, which is commonly referred to as the "average" last freeze. For Lansing, the last freeze date indicated on the "Average" chart is approximately 5/07, or May 7. Another way to think of these three values is a range of dates (90th percentile to the 50th to the 10th percentile), which should statistically encompass 80 percent of all last freeze dates (100% minus 10% at each end of the range of all possible dates).

As is the case in the fall, it is nearly impossible to project very far in advance when the last spring freeze may occur with any accuracy. However, at this point in the season, about the only meteorological way to experience a widespread frost/freeze in central and southern parts of the state is through the occurrence of a "Hudson Bay" high pressure area, which occasionally move south along the western edge of Hudson Bay and into the Great Lakes with a large area of abnormally cold, dry air. This situation caused the Memorial Day freeze in 1992 and many other such past events. As a result, agricultural meteorologists watch carefully for signs of this pattern in the spring.

The accompanying table serves as an Oriental fruit moth (OFM) degree day model providing the predicted egg hatch time periods for all three generations of OFM. Please note that this model is base 45. Treatment thresholds based on trap catch, as with codling moth, have not yet been developed. Where warranted, first control measures should be timed for the beginning of egg hatch for each generation. Where necessary, additional control measures should be applied according to the expected residual effect of the chosen control material.

Source: derived from peach data, Hull & Krawczyk. 2001. Penn State University, Penn Fruit News Vol 81(2):23-36

A Section 18 emergency exemption has been issued by the EPA for use of Topsin M WSB (thiophanate methyl) in blueberries in Michigan and six other states (Connecticut, Indiana, New Jersey, New York, Ohio, and Pennsylvania). The effective period runs from May 6 until September 30, 2002. This emergency exemption was requested to fill the void left by last year's withdrawal of Benlate by DuPont. Many growers have used up their Benlate stocks and do not have access to additional product for the current season.

Topsin M is in the same chemical class (benzimidazoles) as Benlate and breaks down to the same active ingredient. Topsin M and Benlate are practically similar in their spectrum and efficacy of control. Topsin M has been used for years in strawberries and other crops, but was not labeled for blueberries. Cerexagri, Inc., the company producing this material, is currently working on obtaining a full registration for Topsin M use on blueberries. Growers who want to use this product should be in possession of a copy of the special label at the time of use.

Target diseases on the label are mummyberry, Phomopsis twig blight and canker, Fusicoccum canker, anthracnose fruit rot, and Botrytis blossom blight. The recommended application rate is 1 lb/acre, and a maximum of three sprays (3 lbs product) may be applied (this also includes Benlate). Applications may be started at green tip and repeated on seven to ten day intervals. However, use of this fungicide will be most appropriate in the period from pink bud through early fruit development, as the Phomopsis, anthracnose, and mummyberry pathogens are all active at that time.

Topsin M should be used in combination or rotation with non-benzimidazole fungicides, such as Ziram or Captan, to reduce the risk of resistance development. The pre-harvest interval is seven days.

Insect activity over the last week includes first trap catch of the American plum borer, Oriental fruit moth (OFM) and grape berry moth, and continued flight of the redbanded leafroller, green fruitworm and spotted tentiform leafminer. We are setting OFM biofix as May 3, which will put egg hatch out approximately 150 GDD from Friday's accumulation of 230 base 45 since March 1, 2002.

First generation OFM larvae almost solely infest terminals, causing flagging damage on peaches and apples. Leafroller larvae are actively feeding in orchards at this time. Note that since the obliquebanded leafroller (OBLR) overwinter's as second or third instar larvae, they will be slightly larger than the newly hatching fruit tree leafroller (FTLR) larvae, which overwinter as eggs.

Redbanded leafroller overwinter as pupae and egg hatch occurs mid- to late apple bloom. Redbanded leafroller larvae are distinctive from the other two species in that their head, thoracic shield and body color are all light green, whereas OBLR and FTLR larvae have black (or brown) head and thoracic shields. These leafroller species can be found not only in pome fruits, but also stone fruits and blueberries. Growing degree days for the Fennville TNRC are 317 base 42, 249 base 45 and 166 base 50 since March 1, 2002.

Last week's cool weather with highs in the low 50s and lows in the 30s slowed plant growth. There was a light frost on Friday, May 2 that caused little damage. The weekend was warm and dry, plant growth was rapid and apples moved quickly into bloom. Warm (60° F) widespread rain fell Monday (May 6) and resulted in the first major infection period for most fruit diseases. Warm, wet conditions are forecast for the week with clearing and a possible frost by Friday (May 10). The weekend is forecast to be cool. Soil moisture levels are good and soil temperature is 60° F.

In tart cherries, there are lots of green leaves out and Monday's rain was an infection period for cherry leaf spot. Brown rot blossom blight is seldom a problem in tart cherries, so cherry leaf spot is the more important disease. Cold windy weather last week probably hurt pollination. Fruit set is hard to determine. We should have a better idea in two weeks as the fruit comes out of the shuck.

Damage to the sweet cherry crop was fairly severe. Trees that still have crop usually only have fruit on the top portion of the tree. Sweet cherry growers need to worry about brown rot at all times. Monday's rain was an infection period for brown rot.

Pears need protection against pear scab.

In strawberries, spittlebugs are out.

Cool weather over the last week has slowed our spring growth and development back into the "normal" time frame. We did experience a moderate to hard frost on Saturday morning (May 4). This would of caused injury to unirrigated strawberries and may have damaged tree fruit in some isolated locations. Rain on Monday, May 6 resulted in the first major infection of many tree fruit diseases. With warmer and possibly wet conditions forecast later in the week, we may be experiencing additional infection periods. Soil temperatures have now jumped into the high 50's most days. Soil moisture is adequate to good. Subsurface soil moisture conditions are below normal however.

Pears are at bloom, with no major pest problems to report as of yet. There is a distinct lack of pear psylla this year.

Peaches are in the shuck. Oriental fruit moth flight has already been noted in the apple section.

Raspberry leaf development continues for summer bearing plantings. No flower buds are visible at this time. Fall bearing plantings are now up to three inches of new growth. I wrote in last week's Fruit CAT Alert that there wasn't much winter injury on summer raspberries this year. Since that report, I have visited a few farms and talked to a few others that have had extensive amount of winter injury on Canby. It appears that the winter injury occurred late in the fall last year. Some plantings are killed back nearly to the ground.

Degree accumulations are running near to slightly above normal for much of the Grand Rapids region.

I caught a fair amount of apple scab spores from the rains that started the morning of May 6, which is the highest catch so far this year. We are at about 40 to 50 percent spore maturity and have a long way to go. The rain started Monday morning (May 6) in the region. It did dry off for a while, but heavy dew moved in through the evening and into this morning in the Sparta and Fremont areas. Some areas may have even seen a brief shower overnight. I have continued the wetting event in light of this additional moisture, which has changed this rain event into an infection for some areas.

For Sparta, rain started at 9:20 AM on May 6, it was wet for six hours, dried off for five or six hours in the afternoon, but heavy dew started again and leaf wetness is still a concern. I am keeping the Sparta area infection as still ongoing. So far we have seen 14 hours of wetting with an average temperature of 60 degrees and 0.60 inches of rain recorded. According to Mills, this is a moderate infection for the Sparta weather station.

For Belding, rain started at 10:15 AM on May 6 and it was wet for less than five hours with an average temp of 60 degrees with 0.18 inches of rain recorded. The weather station in Belding recorded eight hours of leaf dryness time so the wetting period stopped after only five hours, so there is no infection in the Belding area according to the readings from the Belding weather station.

For Fremont, rains started at 10 AM on May 6 and this station is also reporting leaf wetness again through the nighttime hours. So far there has been ten hours of wetting at 59 degrees. This is a light infection for the Fremont area so far.

Varieties that are susceptible to powdery mildew should have materials added to the tank. This can be especially important now that we are nearing bloom.

Fireblight is not a problem right now, but I am starting to run the MaryBlyt model for fireblight. Pears are in full bloom in much of the Grand Rapids area. Some pollinator crab apples are getting near to full bloom also. Early varieties, such as Ida Reds, Macs, Empires and Gala are starting to show some king blooms open. There is low risk for fireblight as of today, and it looks like the temperatures forecasted for the next several days are not going to get much above the 70 degree mark which does not favor FB epiphytic populations from growing. If we get days in the high 70's or in the 80's then FB will become a concern.

We've had the first reports of Oriental fruit moth flying in the area, and numbers are not unusually high. Apple grain aphids can be found on terminals in low numbers. Overwintering obliquebanded leafroller larvae are found in terminals in normal numbers.

The weather is still temperamental. The last frost event of May 4 did not cause major problems. Petals on some of the open flowers have been "burned" a bit, but the pistils are still nice and green with no apparent damage. Warmer temperatures over the weekend resulted in the vegetation "push" and increased insect activity.

Pears are in balloon stage. They will be entering bloom in a day or two so we will need to be on a look out for the potential fire blight infections. No significant damage from the frost events. Pear psylla eggs are still very difficult to find.

Peaches are in full bloom to very early petal fall. There is a possibility for some crop reduction due to less than favorable weather conditions during the pollination. There is a report of a first Oriental fruit moth catch in one of the "hot spots" in Oceana County. We did not have any other OFM catches in the district. I would expect increased flight activity in the next several days.

Plums are in full bloom. Brown rot and black knot are of concern at this time.

Sweet cherries are in full bloom. Tart cherries are just starting to bloom. Weather is conducive to blossom blight and cherry leaf spot development.

GDD42 236

GDD50 119

Cold, wet weather has continued this past week. Rainfall this spring is above average.

With sweet cherries now in early to full bloom in many areas, it is much easier to assess bud damage. In general, sweets are showing less bud damage than anticipated. With the cold, cloudy weather, pollination conditions have not been very good so far.

Any stone fruits with open bloom are susceptible to blossom brown rot. Apricots are in the shuck and are also susceptible. The cool temperatures have kept the brown rot pressure light, but the risk is always there. There are no competent models for the prediction of brown rot, whether it be in the blossom phase or the fruit phase. Blossom brown rot needs open bloom, extended wetness for a period of time, adequate temperatures and prefers high humidity after the bloom is dried off. Tart cherries are not nearly susceptible as sweet cherries.

Apple grower's main concern remains apple scab. The last two days left us with a light to moderate infection period, depending on your area. This infection occurred because the humidity was greater than 90 percent in the dry period between the rains, which was Monday afternoon. Spore discharge was the highest so far this year in the Grand Rapids area. Approximately 20 percent of the spores are mature at this time and could potentially be discharged during a wetting event. This percentage will increase rapidly from now until petal fall. Spotted tentiform leafminer averaged 46 per trap this past week. Warm weather ahead should increase these numbers substantially. An occasional apple grain aphid can be found in apples. No European red mite egg hatch has been reported yet.

Pear psylla adults continue to lay eggs. Growers can still apply oil or Surround as an egg laying deterrent