Northern Michigan FruitNet 2002
Weekly Update
James E. Nugent      Gary E. Thornton       William M. Klein
NW Michigan Horticultural Research Station
Michigan State University

May 7, 2002

Year        2002      2001      2000      1999      1998        1997
GDD42     236       328         384         387        381          184
GDD50    119        166         177         184        170            62

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

GROWTH STAGES
Apple: Red Delicious—tight cluster
Pear: Bartlett – green cluster
Sweet Cherry: Napoleon – full bloom
Tart Cherry: Montmorency –early white bud
Plum: European type – white bud
Grapes: Chardonnay – late bud swell

COMMODITY REPORTS

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% 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% 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. Spottedtentiform 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. 

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 blossombrown 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 are. 

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

APPLE THINNING 2002
Fruit CAT Alert 4/30/02
By Phil Schwallier, District Hort./Marketing Agent, MSUE

Apple thinning is the most important practice an apple grower needs to perform each year. Thinning can be somewhat unpredictable but in general, good results can be achieved by applying thinners under good weather conditions. All thinners work best when applied during warm temperatures (80’s for highs). The 2 to 3 days after a thinner application needs to be warm to achieve good thinning. Also, apply thinners early to achieve good results such as maximum fruit size and increased return bloom. 

Consider applying first thinning sprays at petal fall if warm temperatures are forecasted for the 2 of 3 days following the applications. At petal fall, fruitlets are less sensitive to thinning than at 10mm but some years the weather that occurs at petal fall can be better than the weather at 10mm. Sevin, NAA and combinationswill provide some thinning to good thinning at petal fall timing. Over thinning at petal fall almost never occurs. A big advantage to applying a thinning spray at petal fall is that there is enough time to thin again at 10-14mm if required. Early thinning at petal fall will promote the greatest return bloom and the greatest increase fruit size.

Difficult to thin varieties are prime candidates to consider starting a thinning program at petal fall. As the petal fall stage approaches, evaluate your potential crop. First, consider last year’s crop. A prior year’s heavy crop will be easier to thin this year. Second, has frost damage occurred during pre-bloom and bloom period? If so, reduce the aggressiveness of the thinning application or consider delaying the thinning application altogether. Third, evaluate the health and strength of the bloom, the pollination 

conditions and the bee activity. If these factors are good or excellent, be ready to start thinning at petal fall only if warm conditions are forecasted.

To summarize, thin early, starting at petal fall where appropriate, and apply thinners at the beginning of a forecasted warming trend. Use mild or aggressive thinning rates based on your best judgment of the potential crop set. Use your past year’s experience with your block to help select your program.

Please send any comments or suggestions regarding this site to: 

Bill Klein, kleinw@pilot.msu.edu
Last Revised: 5-07-02