Freeze Damage and Fruit Diseases: Should You Abandon Your Spray Program?

Should you maintain your fungicide program after fruit loss?

After two nights of freezing temperatures (April 10-11, 2012), many apple growers reported some level of crop loss.  Susceptibility to freeze damage results from an early season when apple bloom began as early as mid-March in some areas.  I saw few late-varieties still in bloom earlier this week, but for the most part, apple flowering was complete when cold weather set in.

According to weather reports, temperatures did not get low enough to impart severe crop damage.  However, some growers have already reported losses.  I have been receiving questions regarding fungicide spray schedules for complete and moderate-loss orchards.  Below are a few disease facts to consider before abandoning your fungicide program.

 Apple

·         Fire blight bacterium was not active during the last few weeks due to dry weather.  Now, this cool dry weather continues to suppress the pathogen. 

o   Warm wet weather can reinvigorate the bacterium and raise risk of infection.

§  Dead blossoms cannot become infected.

§  Secondary (rat tail) blossoms can become infected.  These blossoms usually develop later, so monitor bloom.  If risk is high during secondary bloom, apply streptomycin.  Remember, check CougarBlight (see below) for risk assessment in your area. 

§  Shoot/twig blight phase of the fire blight disease can occur after bloom.  Young tender growth is most susceptible. 

·        Twigs and tissue damaged by freeze, hail, etc.  can become infected, especially when temperatures are warm and rainy. 

·         Twigs and shoots are can become infected even when there is not fruit.

·         Consider the level of risk by monitoring CougarBlight. 

o   Copper is a good choice if fruit load is minimal; streptomycin is recommended for fruit-bearing apple. 

o   If orchards have a history of twig blight, Apogee provides excellent control of twig blight.

§  Chances of a 50-75% crop are high, even though some growers fear the worst.  Evaluate crop losses (and non-losses) as soon as possible.  There is a possibility of at least a moderate fruit load this year.  Consider yield when adjusting spray schedules.  Good yields should receive regular spray regimes.

§  Ideally, plant pathologists prefer that growers maintain a sufficient fungicide program, even after complete fruit loss.  However, economics influence growers’ actions, and many growers opt for a reduced-pesticide regime.  Consider the following and assess your particular situation.

·         In the case of complete fruit loss, fire blight disease management can continue with copper sprays, alone. 

·         If yield potential is moderate to high, growers should follow their regular spray program as closely as possible.  This is a decision that should be based on individual situations.   

·         Bactericides should be applied during bloom or during spans of succulent tender growth if fire blight risk is high.  When conditions are wet and temperatures are above 60˚F, fire blight can infect.  

·         Orchards with a history of fire blight should be sprayed on a regular schedule, regardless of fruit load. 

·         Highly susceptible varieties (i.e. Fugi, Gala) require a regular spray schedule, regardless of fruit load.

·         If risk is low, consider that early copper and/or streptomycin sprays should have reduced inoculum, thus far.  However, proceed with caution.

·         Scab will be active, regardless of fruit load.

o   Continue to protect trees from scab.

§  Do not abandon your scab-management spray program.  However, a low-input programs may be considered – again, from an economic standpoint, not a plant pathology standpoint. 

·         Captan is a lower cost alternative that may be considered.

·         Fungicides may be applied at wider intervals if weather is not conducive for disease.

§  A low-input fungicide program this year will probably result in higher-than-normal disease pressure next year.  Be prepared to follow a strict spray schedule in 2013.

·         Growers should consider potential yield when evaluating low-input spray programs.  This is a decision that should be based on individual situations.

Peach

o   Even in the event of fruit loss, disease management, at some level, is required.  Protection of this season's foliage will positively influence next year’s fruit.

§  Monitor foliar and twig disease outbreaks carefully.

§  A low-input, lower cost spray program can include captan + sulfur. Growers should consider orchard history and weather conditions before selecting a low-input disease management program.

§  Alternatively, wider spray intervals may be suitable if weather is not conducive for disease.

Grape

o   Good disease management programs in the past, as well as during dormant and pre-bloom fungicide applications this season, should reduce fungal inoculum. 

§  Healthy vineyards that suffered complete fruit loss should not have severe disease problems. 

§  Vineyards with high to moderate fruit loss may opt for cheaper fungicides or wider spray intervals.  See above comments for apple and peach.

§  Monitor downy mildew infections in all vineyards.  Those with complete or moderate fruit loss may opt to treat downy mildew outbreaks instead of maintaining a preventative program.  As discussed above, this is not recommended, but economics may influence growers’ decisions. 

All growers should be utilizing UK’s Cougarblight predictive system for fire blight risk in apple.

It is extremely easy to use, as growers should first click on http://wwwagwx.ca.uky.edu/plant_disease.html. Next, click “Fire Blight” on the left side of the screen. Choose the weather station that is closest to the orchard by clicking the arrow under “Station” in the center of the screen. There are a few options below, such as the history of fire blight in the orchard. Finally, click “Submit Choices.” The next screen will describe the risk of infection in the orchard.