Vermont Apple IPM Alert

L.P.Berkett, IPM Specialist

May 2, 2000


Stage of Bud:   Bud development was slowed with the cooler weather last week. Sites that we are monitoring around the state are at Half-Inch Green to Tight Cluster. [However, with the warmer temperatures predicted for the latter part of this week bud development should progress more quickly.]

Disease Management

Apple Scab -- The following chart contains the estimated DD accumulated as of 4/30/00:

2000 Estimated Degree-Day Accumulation (Base 32F,

from McIntosh Green Tip) and Cumulative Ascospore Maturity for Selected Sites






Shoreham (GT=4/15)

New Haven (GT=4/08)

Bennington (GT=4/05)

Saxtons River










5-12 %




We are getting closer to the "accelerated phase" of ascospore maturity which extends from 300-700 DD. The "accelerated phase" is a time of HIGH RISK because there is more susceptible tissue out there and the percentage of the season's ascospores that are matured and ready for discharge builds up rapidly. However, this is not to say that the time before the accelerated phase is not important; on the contrary, in orchards with moderate to high overwintering inoculum, infection periods during the lag phase are very important and should be covered. Whereas, in orchards which have very low levels of overwintering inoculum (as determined by an autumn assessment), the first few infection periods during the lag phase can be ignored without expecting an increase in the incidence of scab. During the accelerated phase, all orchards must be on alert and address infection periods.

So far, it appears most areas of the state had an infection period over April 22-24. Depending on the shower patterns and how quickly the leaves dried off, the rains during the rest of last week may also have turned into additional infection periods in some parts of the state. If you do need to come in after an infection period with a fungicide, note that, in addition to the SI materials, the 'stroby' fungicides have approximately 96 hours of post-infection activity. [Please see the March and April issues of the Vermont Apple Newsletter for further information about these new fungicides.]

As always, it is wise to scout your orchard at the earliest time you would expect to see lesions developing from early infection periods. Table 4 on page 8 of the 2000-2001 New England Apple Pest Management Guide lists the days from infection to first lesion appearance at different average temperature.

Powdery Mildew -- Last year's dry, warm weather was favorable for powdery mildew development and thus, we potentially had more overwintering inoculum. However, the fungus primarily overwinters in apical buds and winter temperatures of -11 F or colder can significantly reduce overwintering inoculum (i.e., it is thought that infected buds are weakened and are more easily killed by cold temperatures). So, those cold temperatures in January were beneficial in terms of potentially reducing mildew inoculum. Plus, the cool, damp weather we have had this spring is not favorable for mildew development. The bottom line is that, to date, we are starting the season in a better position than we potentially could have been in had we not had the cold temperatures this winter or had we had more favorable weather this spring. As the buds move into the pink stage, you should be looking for primary inoculum sites -- these are surviving buds in which the fungus has overwintered. As the tissue expands, the mycelium will grow, covering the leaves and shoots. Conidia produced by the fungus give the tissue a "powdery" appearance. Conidia will be wind-blown from these sites and cause infection on other tissue. Please see the 2000-2001 New England Apple Pest Management Guide for fungicide options if you are concerned about the level of primary inoculum that has survived, particularly in blocks containing susceptible cultivars such as Ginger Gold, Cortland, Gala, Paulared, Idared, Rome Beauty, Granny Smith or Jonathan.

Fire Blight: Last year, fire blight infections were severe in at least five orchards in the state. I suspect many more than these had a few strikes here and there. That translates into an increase in fire blight risk this year. This is a sporadic disease in Vermont but you must be ready for it because it can be devastating. The 2000-2001 New England Apple Pest Management Guide, pages 22-24, gives a good overview of disease management including information on predictive models. In general terms, warm (65-85F), wet and humid weather during bloom favor blossom blight infections.

Arthropod Management Update

2000  Estimated Degree-Day Accumulation

(Base 50F, from Jan. 1) for Selected Sites




Shoreham New













Based on estimated DD, Leafminers and Tarnished Plant Bugs should be active. At the UVM Hort. Res. Center, we had a tremendous jump in LM activity since last week and are over threshold in some blocks. TPB captures are next to nothing so far in our monitored blocks.

ERM egg hatch is estimated to begin at approximately 100 DD. Oil application for ERM management is still an option for many orchards in Vermont but it should not be applied past tight pink.

Coming Events:

Early Pink:  Place EAS traps in orchard

Late Pink:   Check TPB and LM Red Sticky Traps to determine if thresholds have

been reached

Bloom:   Place CM pheromone traps in orchard.  Begin to inspect fruit for PC injury during late bloom


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