Monday, April 30, 2012

Disease-Resistant Knockout Rose Susceptible to Rose Rosette Disease

Disease-Resistant Knockout Rose Susceptible to Rose Rosette Disease

Remember when Knockout rose was the answer to our rose disease problems?  Suddenly, they were everywhere.  I, too, am guilty of mass plantings in both residential and commercial settings.  Then came rose rosette disease; Knockout was susceptible.

During the past 10 years or so, rose growers have struggled with this previously mysterious affliction.  The disease was first described in the 1930’s but it began to spread through our area in the 1980’s.  With the introduction of the Knockout series, roses became popular again.  Consequently, with more roses came more extensive disease spread. 

Rose rosette disease was first diagnosed on Knockout rose in Kentucky in 2009.  Since then, the UK Plant Disease Diagnostic Lab has witnessed a rapid increase in incidence, especially with the disease-resistant Knockout.  This year seems consistent with that trend.

Symptoms:  Disease symptoms vary with rose cultivar, but combinations of all or some of the most distinct symptoms are used for diagnosis. 

·         Stem bunching or clustering, witch’s broom

·         Elongated and/or thickened canes

·         Bright red leaves and stems

·         Excessive thorniness, small red or brown thorns

·         Distorted or aborted flowers

·         Under-developed or narrow leaves and/ or distorted canes

·         Dead or dying canes, yellow or brown foliage, dwarfing or stunting

Causal Agent:  Causal agent of rose rosette disease is a virus classified in the newly described genus Emaravirus. It is systemic and occupies all tissue with a plant, although only some plant parts may be symptomatic.  This pathogen is not spread by pruners or other mechanical means like some common viruses, but it is readily transferred onto rootstocks through grafts.  Due to high demand of Knockout roses, mass production has led to spread of diseaed roses throughout industry. 

In the landscape or garden center, disease is spread by a small mite, the rose leaf curl mite that resides in axillary buds.  Mites are transported on insects or by wind currents for up to 100 yards.

Disease Management:  Viruses become systemic in plants, and there is no cure for infected roses.  Thus, diseased plants must be destroyed so that neither the mite nor the virus is spread to healthy plants.  Mite control is difficult if not impossible, and growers may risk destroying beneficial organisms with excessive insecticide or miticide use.  Prevention is the best control for rose rosette disease.  Growers and homeowners should carefully inspect plants before purchase.  If symptoms are present, avoid purchasing roses from that supplier.  As aforementioned, destroy infected plants immediately and never hold them over in an attempt to cure.  Wild multiflora roses are ideal hosts for both the mite and the virus, so consider destroying nearby wild roses as well. 

Tuesday, April 17, 2012

Homeowner Grape Spray Schedule - Low Impact

Home gardeners who are overwhelmed by spray schedules may be interested in this quick publication. Note, this is a low-impact schedule, so high disease pressure will warrant revision. Contact your UK county Extension agent or refer to other UK publications for more detailed information and

Sample Simplified Home Grape Low-Spray Schedule

Dr. John Strang, Dr. Patsy Wilson, Dr. Nicole Ward & Dr. Ric Bessin University of Kentucky Extension Horticulture, Viticulture, Plant Pathology, and Entomology Specialists respectively


Recommendations based on ID-21. For organic options refer to ID-21. This schedule should provide reasonable control in most seasons, but is not a complete schedule to control every pest problem.  It uses a minimal number of products.

Black Rot of Grape (Clemson)


Ø Sulforix – Anthracnose

Bud Swell

Ø Sevin – only if flea beetles are noted eating holes in the buds

New Growth (2-4” long)

Ø Mancozeb* + myclobutanil - for black rot, Phomopsis cane and leaf spot, powdery mildew & downy mildew

Ø Myclobutanil (Immunox) is systemic and won’t wash off.

New Growth (10-15” long or 7-10 days after last spray)

Ø Mancozeb + myclobutanil for black rot, Phomopsis cane and leaf spot, powdery mildew and downy mildew

Pre- bloom (Just before blooms open)

Ø Mancozeb + myclobutanil - for black rot, Phomopsis cane and leaf spot, powdery mildew & downy mildew


Ø Mancozeb + myclobutanil - for black rot, powdery mildew & downy mildew

Post Bloom

Ø Captan + myclobutanil - for black rot, powdery mildew & downy mildew

Ø Sevin – grape berry moth control

Ø Mix all of these in the same spray tank

First, Second third and forth cover sprays (every 10 to 14 days**)

Ø Captan + myclobutanil - for black rot, powdery mildew & downy mildew

Ø Sevin – Spray for Japanese beetles only when they are present.

Ø Check label for PHI and abide by this

*May substitute Captan for Mancozeb throughout this spray schedule

**Spray schedule frequency should be shortened in rainy weather and extended in dry weather.  Early season sprays are very important because this is a protectant spray program.

Homeowner Apple Spray Schedule - Low Impact

Home gardeners who are overwhelmed by spray schedules may be interested in this quick publication.  Note, this is a low-impact schedule, so high disease pressure will warrant revision.  Contact your UK county Extension agent or refer to other UK publications for more detailed information and 

Sample Simplified Home Apple Low-Spray Schedule

Compiled by Beth Wilson, Pulaski County Horticulture Agent, and

Dr. John Strang, University of Kentucky Extension Fruit Specialist


For pictures of growth stages, see pages 18 and 19 in ID-21.

Recommendations based on ID-21. For organic options refer to ID-21.

Dormant (before buds swell)

Ø Fixed copper – fire blight control

Ø Dormant oil – only if you have scale insect problems

Ø These two sprays may be combined if fixed copper is used.

Green tip to half-inch green

Ø Fixed copper (if you did not spray as a dormant spray) – fire blight control

Ø Dormant oil if not used earlier – for mites and aphids

Ø Mancozeb* + Immunox (myclobutanil)  fungicides for control of apple scab and rust

Ø Fixed copper, dormant oil, Mancozeb and immunox can be combined in one spray.

Ø No insecticide needed at this point

Just before blooms open

Ø Mancozeb + Immunox – you will be on a 10 to 14 day spray schedule with this. Immunox is systemic and won’t wash off.

Ø Mancozeb must be re-applied if a significant rain (more than one inch occurs)

Ø Mancozeb has a 77 day pre-harvest interval (PHI), so you MUST stop spraying with this at least 77 days before you expect the apples to be harvested.

Ø Plus Malathion insecticide


Ø Mancozeb – for cedar apple rust and scab control

Ø No insecticides

Ø Streptomycin – Fire blight control (every 4 days for a total of no more than 4 sprays for maximum control)

After petals fall

Ø Mancozeb + Immunox

Ø Malathion – for insect control

Ø Mix all of these in the same spray tank

Every 2 weeks after

Ø Use mancozeb as long as you can per the 77 day PHI, mix with malathion or permethrin (rotate these 2)

Ø Orchard spray – use this after mancozeb can no longer be safely used.

Ø Orchard spray contains captan (fungicide) and malathion usually, no need to mix in additional insecticide.  Add Topsin M (thiophanate-methyl), another fungicide for powdery mildew and improved sooty blotch and flyspeck control

Ø Check label for PHI and abide by this

*May substitute Captan for Mancozeb throughout this spray schedule

Monday, April 16, 2012

What's this spot on my rose? Downy mildew showing up on rose this spring.

There’s a new disease in town.  Downy mildew of rose is showing up across the state.  It has been several years since downy mildew has been diagnosed on rose in KY, but we have already seen multiple cases this spring.  During the past weeks, there have been reports of an “incurable black spot” that is “unresponsive to fungicides.”

The downy mildew pathogen is a water mold, not a true fungus.  Thus, humid greenhouse conditions are ideal for disease spread.  So far, all reports originate from greenhouse-grown roses.  Additionally, retail centers who purchased finished roses indicated that roses arrived with early stages of the disease. 

Symptoms of downy mildew occur primarily on young apical leaves.  Initial symptoms begin as purplish-red irregular spots (photo below).  Rapid spread occurs with high moisture and reduced air circulation.  Within days, lesions coalesce, resulting in leaf yellowing and/or leaf drop.  The fuzzy sporulation of rose downy mildew occurs on the lower leaf surface, but it is typically very sparse and difficult to see.  This season, infection has been diagnosed on hybrid tea roses and some varieties of Knockout rose. 
Downy mildew on rose begins with irregularly-shaped lesions that are purple to reddish in color.

Roses are susceptible to a variety of leaf spots (photo below).  Black spot, the most common foliar disease of rose, begins as circular spots with feathery margins.  While hybrid tea roses are extremely susceptible to black spot, Knockout roses are mostly resistant.  Cercospora leaf spot, common on many shrub roses, produces circular lesions with purplish red edges and light gray or tan centers.  Both of these leaf spots can be confused with early symptoms of downy mildew.
Leaf spots of rose can appear similar.  Black spot lesions are circular with feathery edges (left, photo by J. Hartman).  Cercospora spots have dark halos with light centers (right, photo by A. Windham).  Downy mildew infections cause blotches, not spots (center, photo by D. Hull).

Management of downy mildew requires a combination of cultural and fungicidal controls.  This first step requires reduction of humidity.  Greenhouses should be well ventilated.  Nursery and greenhouse plants should be spaced for sufficient air circulation to promote leaf drying.  Drip irrigation can greatly reduce leaf wetness, as well.  Growers should practice good sanitation, removing fallen leaves and pruning away diseased plant parts, as the pathogen can survive for several weeks on debris.  Fungicides registered for use in commercial greenhouses and nurseries include Aliette, Banol, Segway, Stature, and Subdue MAXX.  Homeowners may apply copper + mancozeb.  Refer to labels for rates and application intervals. 

Thursday, April 12, 2012

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.


·         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.


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.


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 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.