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Disease Observations for the 2012 Season

-Norman Lalancette, Tree Fruit Pathologist
All plant pathology field studies at the Rutgers Agricultural Research and Extension Center in Upper Deerfield have control treatment trees as part of their experimental design. These control trees, like all other treatment trees in each cultivar block, receive the necessary insecticide applications to combat insect pests. However, unlike the various treatment trees, the controls do not receive any fungicide or bactericide applications. Thus, disease levels on control trees are excellent indicators of the disease pressure in any given season.
Sporulating blossom blight canker in peach

Some observations for this season so far:

Rusty spot 

Powdery mildew fungi prefer drier weather for infection and survive in greater numbers during less severe winters. Consequently, the dry conditions experienced this spring combined with a moderate 2011-2012 winter has resulted in record high levels of peach rusty spot. For example, 97% of fruit on our moderately susceptible ‘Encore’ control trees had rusty spot infections by mid-May, and current lesions densities are approaching 5 lesions per fruit on average. In comparison, only 35% fruit infection was observed on these trees in 2011 with an average 0.39 lesions per fruit. Other cultivars show similar increases. So far, non-sprayed ‘Bounty’ and ‘GaLa’ have 99% and 72% fruit infected this year in comparison to 55% and 11% fruit infection in 2011, respectively.

Optimum fungicide timing for rusty spot consists of sprays at petal fall, shuck-split, first cover, and second cover. This four-spray program provides effective control in most seasons. However, past research indicated that in some years only three sprays are needed, while in other seasons five sprays are best. Clearly, 2012 is the latter case; a third cover spray is recommended. Ideally, fungicide coverage is needed until initiation of pit-hardening, which generally occurs mid-June, but may well occur sooner this year given the early bloom. Rally 40WSP is the recommended fungicide with rates of 3, 4, and 5 oz/A for cultivars of low, moderate, and high susceptibility, respectively.

Blossom blight 

Record levels of blossom blight canker development have also been observed this year on our non-sprayed trees. Our Encore and Autumnglo control trees have 88% and 94% of their shoots infected, with an average 3.4 and 6.1 cankers per shoot, respectively. This is an incredibly high amount of infection! Typically, we observe 10% to 20% infected shoots per tree with an average 1 or fewer cankers per shoot. Although the spring period was generally dry, the extended bloom and light rains during the later petal-fall stage was most likely the time of infection. Overwintering inoculum levels produced on our mummies, of which we have many, may have also been higher than normal due to the moderate winter conditions.

Although blossom blight infection pressure was severe, many of our experimental and newly released fungicides provided high levels of control, some yielding 100% control. The Vangard standard at 5 oz/A, however, only yielded 66% control this year, whereas in past years it has provided 85-100% control. Note this treatment was located in the Autumnglo orchard, which had the highest amount of disease pressure. Nevertheless, commercial orchards should be scouted for presence of blossom blight cankers since these cankers are an important inoculum source for preharvest brown rot fruit infection. Keep in mind that low levels, say 5-10% shoot infection, are more difficult to detect yet still provide plenty of inoculum. A photo of a sporulating blossom blight canker and additional information can be found in the March 21, 2012 Fruit Edition issue of Plant & Pest Advisory (Vol.16, No. 33).

Bacterial spot

We have yet to perform our first bacterial spot disease assessment, but casual observations on fruit in all of our blocks indicate that disease pressure is above average. Of course, as would be expected, fruit lesions are readily found in our non-treated highly susceptible O’Henry trees. However, a good number of fruit in our moderately susceptible Bounty, GaLa, and Encore orchards have been observed with lesions; these blocks are not sprayed with bactericides. In particular, bacterial spot is not often seen on Encore fruit, so it’s presence in this block is certainly indicative of disease-favorable conditions for this season, whether it be time of infection or inoculum levels.

Copper-based bactericides should continue to be applied as a basic program for high and moderate susceptible cultivars. The standard, Kocide 3000, should be applied at 0.75 to 1.5 oz/A in the highest possible volume for thorough coverage; the label allows a maximum of 6 covers. Weekly spray intervals are recommended, but shortening or lengthening the spray interval according to rain events makes the most sense. The antibiotic oxytetracycline (FireLine, Mycoshield) is best employed immediately before a possible rain [infection] event, integrating it into the copper program.