IPM News

    Lorraine P. Berkett, IPM Specialist


Disease Management Update

Apple Scab

If you have scab in your orchard, it is time to evaluate why.  If you thought the timing of your fungicide applications was 'right on' in covering the infection periods we had, the next items to check are:   Your  fungicide rate - was it too low ?  Did you base your rate on an incorrect  tree row volume and dilute gallonage calculation?  Was there poor coverage because of alternate row spraying?   Was there poor coverage because an  application was applied in less than ideal conditions, i.e., windy conditions?   For example, in one orchard block we visited with mixed sized trees, scab was present on foliage on the larger trees  - - timing was right in terms of when infection periods occurred but sprays had been applied on an alternate role schedule and it appeared that the amount of fungicide was just not enough to be effective on the larger trees.  In another orchard that we visited that has had problems with scab in the past, the orchard is the "cleanest" ever, even on very big, thick trees  - - here the orchardist this year increased the amount of water applied per acre and increased the rate of fungicide.

The good news is that the hot weather (80+F)  we have had over the last few weeks reduces the viability of scab conidia in lesions present in the orchard.  As was mentioned in the IPM Alert on June 2, conidia production in lesions tends to decrease after about 4 weeks, with the highest production during the first two weeks after the lesion appear.   Assuming that the last primary scab infection period was on June 1,  all the primary lesions should be visible in orchards by now.   Hopefully,  primary scab was effectively managed in most orchards around the state.  It is important to continue to look at younger leaves on terminals to confirm that you do not have any secondary lesions developing before you increase spray intervals and/or decrease fungicide rates.

Sooty Blotch and Fly Speck Disease

These "summer diseases" are caused by a complex of fungi and result in surface blemishes on the fruit that lower fruit quality.  Sooty blotch fungal colonies appear as olive-green smudges on the fruit.  Flyspeck fungal colonies look like well defined "fly specks" which appear in clusters of 2-50 black specks on the fruit.  These diseases gradually develop during periods of high humidity. Infections can occur on fruit as early as 2-3 weeks after petal fall, however,   symptoms may only appear 20 to 25 days after infection.  Optimal conditions for fungal growth are temperatures between 60-75 F and 95% humidity.
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Cultural practices such as proper thinning, summer pruning, and mowing can aid in reducing the incidence of sooty blotch and flyspeck by affecting the micro-climate and increasing air circulation. The fungi can grow on a wide range of host plants that surround the orchard including brambles.  Thus, bush-hogging fencerows or ditchbanks may aid in reducing sources of inoculum. 

Of the two diseases, sooty blotch is more easily managed than flyspeck.  If you see sooty blotch on your fruit it is an indication that at some point past petal fall you had very little fungicide residue .   A key time for infections for both sooty blotch and flyspeck is from petal fall through about the third week in June.   Table 14 on page 64 of the 1998-1999 New England Apple Pest Management Guide has information on fungicide options for these diseases including how long fungicide residue is expected to be effective, the maximum rain before another application is needed, and an estimate of the maximum days from the final fungicide application to harvest that you can expect various fungicides to be effective. 

It should be noted that these diseases are more of a problem the farther south you go in New England, down through the Hudson Valley, through the mid-Atlantic states, into  the southeastern states where all the apple crop would be affected each year if fungicides specifically targeted at these diseases were not used.   Severity in Vermont is usually not comparable to the southeast.  However, the following picture will  give you an idea of how severe sooty blotch can get in Vermont when no fungicides are applied after 14 days past petal fall during one of the wettest years on record (1998):

Example of  severe sooty blotch in 1998.  Over 50% of the fruit surface was affected.

Terminal on right has powdery mildew.  Note the smaller leaves on infected shoot.

Powdery Mildew

If you did not use SI fungicides for apple scab this year, do not be surprised if you have powdery mildew on the 4-5 youngest leaves on the end of vegetative terminals.  Even if infections look severe, the time has past to treat with fungicides (see the May issue of the 1999 Vermont Apple Newsletter).  Once apical bud sets on the terminals and the leaves harden off, they are no longer susceptible to infection.  New infections will only occur if the apical bud breaks and there is a spurt of new growth - these new, young leaves may become infected but the value of trying to manage these new infections is very questionable.  If one wants to look on the bright side, mildew infections can reduce leaf surface which may allow more light penetration into trees with dense canopies  !!

Blossom with fire blight.  Note the darkened fruit.

Fire Blight  -  A Cautionary Note

We have seen a sample of blossoms and shoots from a Vermont orchard that had fire blight symptoms -- the blossoms died back, young terminals developing from fruit cluster wilted and resembled a 'shepherd's crook',  leaves and developing fruit looked 'water-soaked' and were oozing clear to amber colored, sticky droplets of bacteria.    It serves as a warning to check your orchard, particularly young, susceptible cultivars, for symptoms of fire blight.  On page 20 of the 1998-1999 New England Apple Pest Management Guide, there is a description of the "ugly stub" method for cutting the fire blight strikes from the tree.   Particularly on young trees, it is important to cut out strikes before extensive necrosis occurs.  It is always advisable to prune out infections on sunny, dry days when rain is not forecast.    Hopefully, we will only see at the very most an occasional strike here and there around the state this year.

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