Apple pest report

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AppPestRept-2005-06-10.htm

The estimated last scab ascospore release was June 8 for Sanford, and June 10 for Gorham, Monmouth and Old Town.

The infection periods on June 5 - June 8 were substantial in all locations. Rain on June 10 brought a final substantial primary scab infection period for Gorham, Monmouth and Old Town, but it does not appear to have counted for much in Sanford.

So is scab over? Primary scab ascospore releases are done, but if any of those spores successfully caused infections, then lesions (scab colonies) will begin producing spores to spread secondary scab starting about 9-17 days (generally about 14) after the infection started.

That's why if there were gaps (such as inability to recover washed-off fungicide in a timely manner during the May 15-30 rainy spell) in your primary scab control program, it is prudent to maintain some level of protection until you can verify that scab has been adequately controlled. If you let fungicide protection completely deplete after primary scab, (figuring that protection for flyspeck isn't needed until later in the summer), then if you discover scab lesions popping up a couple of weeks from now those lesions will have already had opportunity during any rain that occurred prior to detection to spread a whole new generation of scab spores within the tree. And now there are tender young fruitlets exposed to scab infection.

A key difference between secondary scab spores and the primary ascospores is that the secondary scab spores (called conidia) do not travel as far. Secondary scab spreads mainly by conidia washing onto other leaves and fruit in the same or a neighboring tree.

Unless you are confident that your protection against primary scab was rock solid, you should factor scab protection into spray decisions for at least a couple more weeks until you can verify that scab control has been successful.

At a minimum that requires scouting for scab until at least the earliest developing lesions from all of the primary scab infection periods have had time to begin appearing. Lesions from an infection period do not all appear on the same day. Infections that are delayed, but not killed, by a sterol inhibitor or strobilurin fungicide can resume progress after a delay of days, weeks, or even months. And infections on older tougher leaves will also take longer than those on tender leaves that were only a few days old at time of infection.

Complicating matters even more - 1st generation lesions can be hard to detect because they often are located high in the tree, or can be numerous enough to cause problems with secondary scab, but still sparse enough to be hard to find. A few years back my “scab wrap up” check at Highmoor was about a week before that estimated date and things looked pretty good. They didn’t look so good a week later with a surprising amount of scab that seemed to have come from nowhere. Of courses, it was there all along, but just hadn’t revealed itself at the time of the earlier check.

All of this is my case to suggest that you consider maintaining some degree of protection until 2nd generation lesions have at least had a chance to begin appearing if you have a low tolerance for scabby fruit (i.e. a commercial grower) and if you have concern about potential breaks in scab protection this spring. There is good reason for many growers to be concerned given higher than average inoculum coming into the season combined with the weather we had. While the weather this May was lousy for mammals, insects, mites and spray coverage, it was perfectly lovely if you were a scab ascospore having landed on a nice wet unprotected apple leaf.

You want more, try this! Scab from early season infection periods was showing up in abundance by Monday June 6 in an unsprayed block. The current forecast shows another rainy week coming. If that forecast holds, then ideal infection conditions will be on available as the progressively more of the season’s primary scab infection potential reaches the time required to start appearing as lesions and spreading secondary scab.

OK, time for some good news. Yes I am suggesting that fungicide protection against apple scab should be maintained right through June. But that doesn't mean intense primary scab season spray coverage is needed. A full dose captan or other protectant fungicide will be keep secondary scab in check for about 14 days or two inches of rain, whichever comes first. So even though I'm arguing for several more weeks of scab protection, that doesn't mean spraying every seven days. To help with scheduling these "in case of secondary scab" sprays, I've added a new chart to Orchard Radar. The Monmouth version is at http://pronewengland.org/content/AllModels/memodel/me-Monmouth-SecondaryScabSpray.htm

At Highmoor Farm in Monmouth for example, full-dose captan that was applied on June 5 should provide protection until around June 16. Coverage is not expected to last a full 14 days because of the rain forecast for June 12-16. By the time that protection wears off on June 16, about 64% of primary scab will have had time to appear as 1st generation lesions, and about 9% as 2nd generation lesions. We'll be scouting to see if scab is showing up.

A renewal of full-dose captan coverage on June 16 will likely provide protection until near June 30, by which time all of primary scab potential will have had time to appear as 1^st generation lesions, and at least 80% of the primary scab infection potential will have had a chance to begin showing as 2nd generation lesions.

If scab still hasn't shown up by then, then it's a judgment call about whether to reapply captan again right away or to consider scab under control and to consider the June 16 as a flyspeck preventive spray that allows a longer interval before having to respray (especially since risk of flyspeck infection with development of 2^nd generation flyspeck spores isn’t expected until around July 15). If we did decide to “coast” at that point, we would do so only with plan for at least weekly scouting until 100% of 2^nd generation scab lesions have had time to appear (roughly July 4).

How much scab is too much? One research paper suggested a threshold of 0.5% of leaves showing scab, i.e. 1 leaf out of 200. That level dovetails nicely with the system Bill MacHardy developed for a different purpose, measuring scab level in September in order to determine if the fall scab level is low enough to allow ignoring the first few infection periods the following spring. That system boils down to checking 100 shoots and fruit clusters for scab. You quickly glance at about 15 leaves at a time. You don't have time to stare at them, and you don't need to. If there is scab there you will see it. It takes about 30 minutes to do a block, including the walking. If you find more than 5 scabby leaves on 100 shoots, then you should consider the need for continuing protection against secondary scab until active lesions deplete their supply of spores. If you find more than 18 leaves, that’s a strong case for secondary scab control.

Shutting down secondary scab takes at least 4 weeks. In a severe outbreak you can begin suppression with back to back sprays about a week apart. In that case, a strobilurin + full dose captan application would be the biggest gun to both suppress subsequent spore development and to protect tender foliage and fruit tissue.

Here is some more good news. The above average temperatures since June 2 has worked against spread of scab. Captan plus temperatures in the mid-80s is ideal for shutting down scab. The warm weather is supposed to continue into early next week.

A check of about 1000 Liberty fruit (which size up early relative to McIntosh and other cultivars and are well liked by plum curculio) in two blocks found no damage on Tuesday June 7 when fruit were still only in the 3-4 mm range. By Friday June 10, Patti McManus at Highmoor found zero damage on roughly 400 small M26 Liberty trees, and 5 fresh cuts on roughly 400 fruit in the larger M7 Liberties close to the woods. So that's just over 1% damage on the unsprayed M7 Liberty trees.

Another good thing about the warm weather is that it means plum curculio season should be relatively short due to rapid accumulation of degree days. Using Highmoor as an example, a full-dose McIntosh petal fall Imidan application on Sunday June 5 should provide protection until about June 15. A renewal on June 16 would provide protection until the projected end of the plum curculio control window around June 29.

Speaking of plum curculio control, the “Extremely Hazardous Substance” listing for Imidan has been revoked and having Imidan in storage no longer requires notifying your local fire department and/or county emergency management program.

Mites don't do well in rainy cool weather, they like it hot and dry. On Tuesday June 7, Laura Miller, the Extension apple scout, and I could not find mites on Red Delicious trees at Highmoor. First generation European red mites, if present , would be quite visible by now. If you find mites on more than 30% of leaves, or if there is an average of more than 1 living hatched mite per leaf, then treatment is recommended. If not, it is unlikely that the mite population will do any measurable harm to the crop or the tree.

Leafminers don’t like rainy cool weather in bloom when they are trying to lay eggs. Nevertheless, the first two weeks after petal fall are an important time for assessing leafminer and mite levels. If control is needed, it is more effective to do so at this time than waiting. The threshold for leafminer varies from 7 to 100 mines per 100 leaves (i.e. average of 0.07 to 1.0 mines per leaf), depending on researcher perspective. I think 0.07 is too low, and I’d be pretty nervous about finding an average of 1 per leaf. The middle ground of about 0.5 mines per leaf seems like a reasonable place to land. Keep in mind that biocontrols that we rarely notice normally function to keep mites and leafminers in check.