Disease Management Update  --  Observations from Visiting Orchards over the last two weeks

Apple Scab:   

What more can anyone say ...  it was a very challenging  spring for scab.    It appears that  in some orchards  scab lesions showed up on the very earliest leaves (perhaps infected during the wet conditions that were present in some parts of the state on April 22-23) and it was a battle all throughout the primary scab season.  In other orchards, it appears the period from May 5-12 was critical because of all the rain and the washing off of  fungicide residue,  allowing  infection to occur before the next fungicide application. 

The one question that is always asked this time of the growing season is:  "Are the scab lesions 'burned-out' ? "     It is estimated that each leaf lesion can produce up to 100,000 conidia.  However,  peak conidia production occurs during the first two weeks after the lesion is visible and production is relatively lower after four weeks.  A number of fungicides can effectively reduce the viability of conidia and suppress production.    Unfortunately,  although parts of the lesion may appear 'burned-out' or dead, conidia production usually continues  through the summer at low levels.     The good news is that although the fruit is highly susceptible to infection through fruit set and it becomes increasingly resistant as it matures.  However, it still can become infected later in the growing season,  but at least  28 hours of continuous wetness are required for infection to occur.   [Note:  The greater the period of continuous, wet conditions, the greater the amount of scab. ]   

What does all this mean?   If you have lesions in your orchard, even if they are old, you should keep your fruit protected with fungicide residue,  realizing that the risk of infection increases if we have extended, continuous wetting periods. 

Foliar Scab on Non-sprayed McIntosh trees.

Cedar Apple Rust  (CAR) --  Around the state, you can see a few foliar lesions in some commercial orchards,  but for the most part, the fungicides such as the EBDC and the SI fungicides that were applied  from Tight Cluster through Petal Fall were effective in preventing infection.  Again, the potential for infection was high given the wet weather.   At the UVM Hort. Research Center, we had an interesting situation develop.   We have Ginger Gold in two separate blocks.   In one block, the first fungicide (Manzate)  was applied  on  Monday,  May 8 (late Pink bud stage) ; in the other block,  the first spray (Sovran) was delayed until Thursday,  May 11 after the rains.   (Between  May 9-10,   approximately 2.76 inches of rain fell and thus, the Manzate application was re-applied on May 10).   Both blocks received a Manzate application a week later during late Bloom and captan at Petal Fall. It is interesting to see that the two blocks have a striking difference in the incidence of cedar apple rust lesions on the foliage.   In the block that received the EBDC before all the rain,  about 3% of the terminal leaves are infected with 1 lesion per leaf;   in the block that received Sovran, 26% of the terminal leaves have approximately  5 lesions/leaf.    (We have not assessed the fruit yet for lesions but suspect there will also be a difference.)

The lesson we learned was that  although we were concentrating on scab and were delaying our first fungicide application in our blocks until Pink because our PAD last fall indicated our orchard was at "low risk"  for early season scab,   we have to be aware of the potential of cedar apple rust in particularly wet years on particularly susceptible cultivars (such as Ginger Gold)  and adjust  our fungicide schedule accordingly. 

Cedar Apple Rust Lesions on Ginger Gold

Quince Rust (QR) --   Given the wet spring,  it is not surprising to see quince rust lesions (dark green areas) on the calyx end of Red Delicious and Cortland fruit in orchards around the state.  The fungus that causes quince rust is in the same genus as the fungus that causes cedar apple rust, but unlike  CAR,  the QR fungus does not cause leaf infections.  The QR fungus overwinters on low-creeping junipers where galls appear as spindle-shaped enlargements on the stems.   These galls swell during wet spring weather and release spores that can infect the apple fruit.   QR is most likely to cause economic damage in seasons when the trees remain wet for more than 48 hours during the Tight Cluster through Late Pink bud stages when the average temperature is above 50F.    Like CAR,  there is no spread of the disease from apple to apple -- what you see now is what you will have for the season. 

Quince Rust Lesion on the Calyx end of Red Delicious.   (Dark green lesion)

Phytophora Crown and Root Rot

Typical symptoms of Phytophora crown  and root rot have been observed -- both in orchard blocks that are considered well-drained and in blocks whose soil tends to stay wet.  Above ground tree symptoms  are similar to that caused by  vole damage -- reduced vigor and growth,  sparse foliage.   Trees can decline over several growing seasons.  Sometimes, the tree will leaf out and then suddenly die -- this situation usually follows a very wet autumn or spring.   If the crown and roots of Phytophora infected trees  are exposed, one can see the diagnostic spongy/soft bark and the orange to red-brown decayed cambium and phloem areas (see picture to the left).
Again,  the rain we had last autumn associated with the hurricane plus the rains this spring provided conditions favorable for infection.   The fungi associated with this disease persist as "oospores"  in the soil.   Under certain conditions, the fungi  produce "zoospores"  that can "swim" in the film of water around soil particles to the tree where they can infect. 

Rootstocks vary in their susceptibility to infection:   M.9 appears to be the most resistant;  M.7 and M.7a are considered moderately susceptible;  and  MM.106 is considered to be very susceptible.    Experience with M.26 and MM.111 is variable;  M.26 can be moderately to very susceptible and MM.111 has been described as moderately susceptible to relatively resistant. 

Management of Phytophora rots involves an integrated approach  that includes:  site selection,  choosing a rootstock adapted to your site,  soil water management,  and,  in  certain situations, fungicide treatment.  Table 6 on page 21 of  the 2000-2001 New England Apple Pest Management  Guide lists characteristics of a number of rootstocks.    Please note that the Guide also provides information on fungicide options, such as Ridomil  and Aliette.   These materials do not kill the fungus but stop it from growing for a time -- giving the tree some time to recover.   They will not 'revitalize' trees showing moderate to severe disease symptoms.   

If you have trees that have poor growth  -- check the base of the tree for any signs of insect (i.e., borer ) or vole damage,  then check the crown and roots for signs of Phytophora rot.  If it is Phytophora rot,  you may want to consider a post-harvest application of Ridomil this fall and a pre-season application next spring as a "stop-gap" measure, remembering that the treatments are not a cure for the disease. 

Sooty Bloch and Flyspeck

As mentioned in the last issue of the newsletter,  based on the assumption that most of the inoculum for flyspeck infections comes from conidia produced on wild hosts surrounding the orchard,  summer fungicides for controlling flyspeck are not needed  until 270 hours of wetting have accumulated from 10 days after petal fall.  At that point, presumably, flyspeck conidia will become available in the orchard perimeter and will begin blowing into the orchard.   [This assumes that  infection by flyspeck ascospores within the orchard was prevented by fungicides applied for apple scab through peak flyspeck ascospore release, which occurs approximately 10 days past petal fall.]  As a point of comparison,  as of July 16th,  approximately 260  hours of leaf wetness had accumulated from May 29  (PF + 10 days) at the UVM Hort. Research  Center.

Summer pruning and mowing  which improve air circulation and drying within the trees are important management tactics for these diseases -- they should not be overlooked as effective management tools.