Apple Pest Report: Tuesday, June 6, 2006
Vol. 14, No. 10
Scab situation
The rain on Thursday-Sunday, May 31 - June 4 brought with it the year's final primary scab ascospore release. This final primary scab infection period may also have been the year's most severe. There was probably a substantial portion of spores available for release, definitely a prolonged wetting, and plenty of young apples at peak susceptibility to infection.
But if your trees had fungicide protection in place before the start of the rains on Thursday May 31, scab protection should have been secure. By the time the steady heavy rain started on Saturday June 3, there were likely few ascospores left to cause infection. This applies even for locations that lag as much as a week behind Highmoor in bud stage. In other words, for any orchard that reached McIntosh 95% petal fall by May 30, primary scab season is now over. That should include all but the farthest northern or downeast locations.
Orchards that did not have adequate protection heading into the May
31-June 4 rains should receive post-infection fungicide treatment with
an SI (Nova, Procure, Rubigan) or strobilurin (Flint, Sovran, Pristine)
as soon as possible. If an SI fungicide is used, it should be
combined with a full rate of a protectant fungicide (a "surface
fungicide" in my lingo, but nobody else uses that terminology) for
forward protection and to prevent resistant scab selection.
The end of primary scab does not mean the end of scab management
Until you can verify that scab control was successful, it is prudent to maintain fungicide protection to prevent spread of scab from lesions that may not be appearing from earlier infections. The fungicide respray guidelines at http://pronewengland.org/AllModels/MEmodel/me-Monmouth-SecondaryScabSpray.htm are based on rain at Highmoor Farm, but may be useful for other locations with similar rain patterns.
The orange bars in the chart below show the expected percent of earlier scab infection potential that has had time to begin appearing as lesions by each date. A scab lesion consists of thousands of conidial spores. Rain falling on 1st generation lesions washes those conidia onto other foliage and fruit to spread scab infection if those tissues are not protected. The percent of scab infection potential that has had time to develop 1st generation lesions, get rained upon, and then had enough time for 2nd generation lesions to appear, is shown in purple.
The chart indicates that the percent of scab infection potential becoming visible has increased rapidly in the past week, and that by today, June 6, about 80% of season's potential 1st generation scab lesions has now had time to appear. The chart estimates that only about a third of this year's primary scab infection potential has had time to appear as 2nd generation lesions by June 6.
The chart also forecasts that by June 15, all of the season's 1st generation scab lesions will have had time to appear, and that by then about 80% of the 1st generation lesions will have had time to generate visible 2nd generation lesions.
So if you haven't already started, now is a good time to begin scab scouting. If active scab lesions are present, you want to not to know that as soon as possible so that you can factor that into your spray decisions.
Ideally, you could stop checking for scab after 100% of 1st generation lesions have had time to appear and you find no active lesions. Alas, the world is not quite as simple as we would like it to be. The 1st generation lesions can be sparse and hard to find. They are most likely to occur in tops of trees. And because each 1st generation lesion can produce 40,000+ conidia over a four week "lifespan", it doesn't take too many of them to create an economically significant degree of secondary scab risk. A thorough check once a week for the next three weeks will provide information on all of the 1st generation and most of the 2nd generation lesion potential.
To really be sure, scab scouting should continue until the end of June - early July (too early to forecast the date) to confirm that the last 20% of infection potential has not spread as 2nd generation lesions. But don't let the "perfect be the enemy of the good". If five weekly scouting visits is more drudgery than you will realistically do, note that the highest priority and most value is for scouting this week and the following two weeks for a total of at least three inspections.
What to do if you find "too many" lesions? "Too many" is difficult to define and is situation specific. For commercial wholesale marketing of fruit grown on semi-dwarf trees, a threshold of 5 scab-infested leaves per 100 shoots or fruit clusters is suggested as justification to increase scab protection with two fungicide applications at about a 7-10 day interval to provide solid protection against conidial infections during the first few weeks of heaviest spore production by active scab lesions.
In addition to bolstered protection to shut down a scab epidemic, some fungicides also have the ability to reduce conidial spore production from treated lesions. The postsymptom scab fungicides include the strobilurins (Flint, Sovran, Pristine), Topsin M, and Syllit.
Even though captan is not rated as having this postsymptom activity, I've seen some data that suggest that it also helps in this regard. Even if that is not the case, captan used alone at a full dose is pretty good at shutting down a scab epidemic because it effectively prevents the next generation of lesions. This is especially true it hot dry weather follows the applications. "Hot" in this case means daily highs of 84F or higher.
If the outbreak is severe, i.e. more than about 20 scab-infested leaves per 100 shoots/clusters, or if your market sensitivity is high, then you might consider combining captan with one of the postsymptom fungicides to get the best of both activities - spore reduction and protection against new infections. While the strobilurins, Topsin M, and Syllit all function as good protectants on their own, all of these materials also share the characteristic of being subject to selection for resistant strains of scab, especially when used on sporulating lesions. In my humble opinion, if the degree of scab pressure is high enough that two back-to-back, short-interval captan applications are not considered adequate to keep the scab in check, then there is too much scab pressure to use a strobilurin, Topsin M, or Syllit alone without captan for additional protection and to forestall resistance.
Fire blight
Except for young trees that still had late blossoms open on May 31, I don't think there was any fire blight blossom infection potential in Maine this year. But if there were any trees with open blossoms on May 31, the weather on that day was suitable for fire blight bacteria (if present) to infect those blossoms. If infection did occur, you would see wilting and discolored tissue on the fruit clusters becoming apparent by June 18, and probably earlier with close inspection.
Any fire blight strikes on established trees should be pruned out as soon as found, with sterilization of blades between cuts and certainly before using the same tools on uninfected trees. First year trees with fire blight infection are probably too much at risk for further investment of time and effort. In other words, they should be replaced.
Insects and Mites
The
threat of
plum curculio
egglaying will require
that insecticide protection last until about June 23 at Highmoor Farm
and vicinity. Many Maine apple growers are reapplying insecticide
on Monday and Tuesday, June 5 & 6 following the weekend rains.
Those applications should provide protection against plum curculio until
about June 15-16. Whether that is close enough to the
model-estimated date for the required duration of plum curculio
protection depends on local pressure and market sensitivity. And
of course, the actual end date for effective protection from those
applications depends on how much rain falls in the coming 7-14 days.
The plum curculio degree day model is online for Highmoor Farm at
http://pronewengland.org/AllModels/MEmodel/me-Monmouth-PCTable.htm
If you are debating the need for a follow-up insecticide application for plum curculio, a perimeter-only application after a previous full block application will likely provide adequate protection.
Time is getting short to make 1st generation Leafminer mine counts. They will soon progress to the tissue-feeding stage when mines are visible on the top side of the leaf and insecticide control is less effective. Thresholds and scouting procedure were described in the May 26 Apple Pest Report.
European red mites are heading toward their annual 1st generation die off. Low mite counts in the week of June 12-19 can be misleading. As 1st generation adults die before 2nd generation eggs hatch, the population can appear to be low and then surprisingly bounce back a week later.
The mite treatment threshold until June 15 is if more than 30% of middle-aged fruit cluster leaves have living, hatched mites present, or if there is more than an average of 1 mite per leaf. If you check 40 leaves and find 5 or fewer with mites, then it is safe to assume you are below threshold. If you find 23 or more infested leaves, then treatment is probably needed. The Apple Pest Scout Card has charts to help with field sampling. Having the chart in your pocket makes mite scouting easy and efficient.
Lest I be accurately accused of unrepressed verbosity at a busy time of year, I am ending this issue here. But for those of you who like to know the truth, the whole truth, and nothing but, this week's edition has supplemental information on not-scab leaf spots that you may find very useful in scab scouting.
Sincerely,
Glen
more Scab (or not?)
"Is it scab or not scab, that
is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fungi,
Or to take arms against a sea of rain..."
When checking for scab, there are many other spots on leaves that can be
confused with scab. Here is a picture of what scab lesions
typically look like. (Scab photos by Dr. William MacHardy, Univ.
New Hampshire). The lesions are not always this obvious,
especially as they first emerge.
Then there are all the other things that cause leaf spots at this time of year. Dr. David Rosenberger of the Cornell University Hudson Valley Lab wrote the following as two articles in the May 30 and June 5 editions of the Scaffolds Newsletter (http://www.nysaes.cornell.edu/ent/scaffolds/2006/). While it makes for a long article and a too-long newsletter, I combined the two articles so that you have all the discussion of "leaf spots that are not scab" in one place.
CAUSES OF EARLY SUMMER LEAF SPOTS - Dr.
Dave Rosenberger
"Leaf spots on fruit trees are caused by a wide variety of pathogens and
abiotic factors. Most growers can identify typical leaf lesions caused
by apple scab, cedar apple rust, powdery mildew, and cherry leaf spot.
However, when leaves develop small, nondescript brown leaf spots or
small shot holes, even experienced plant pathologists often have
difficulty identifying the causes.
Fortunately, the nondescript leaf spot diseases in the Northeastern United States rarely cause economic losses, even when their appearance temporarily disfigures the tree canopy. The fungi causing apple leaf spot diseases either do not have secondary cycles on leaves or they are easily controlled with fungicides and appear only when fungicide protection is disrupted by extended spring rain events. Abiotic leaf spots that develop shortly after petal fall are often attributable to agrichemical mixtures that have caused localized phytotoxicity.
Following are some of the most common causes of early season leaf spots and clues for determining their causes. This article focuses on leaf spots that may appear in May, June, and July. Leaf spots with other causes and symptoms sometimes appear during August and September, but they will not be discussed here.
Frog-eye leaf spot, caused by Botryosphaeria obtusa, is the stereotypical leaf spot disease on apples. Frog-eye leaf spots are round, dark brown spots, 2-5 mm in diameter, with an almost black border and a tan center (Fig. 1).
Fig.
1: Frog-eye leaf spot on Cortland showing typical tan lesions dark brown
edges.
Individual leaves may have a single spot or as many as 30 to 50 spots. Frog-eye can usually be differentiated from other kinds of leaf spots by its non-random distribution and its association with nearby inoculum sources. In sprayed orchards, frog-eye leaf spots are usually concentrated in the vicinity of mummified fruitlets that were retained after fruit thinning. Fruitlet mummies can be colonized by B. obtusa and then provide inoculum for infecting the leaves the following season. Spores are dispersed by splashing rain between tight cluster and about second cover. Frog-eye is most common on apple cultivars such as Cortland, Northern Spy, and Honeycrisp, that retain many fruitlets after chemical thinning (Fig. 2)
Fig.
2: Severe frog-eye leaf spot on Cortland showing a fruitlet mummy
(center of photo) that provided inoculum for infections.
However, all cultivars may retain thinned fruit in years when weather conditions fail to promote rapid abscission of thinned fruitlets.
Frog-eye leaf spot may cause premature drop of severely affected leaves, but most damage from frog-eye is cosmetic. The same fungus that causes frog-eye leaf spot also causes black rot fruit decay, but there is no evidence that leaf spots contribute to fruit infection. Instead, the inoculum for fruit infection comes from the same fruit mummies that provide the inoculum for leaf infection. Thus, frog-eye on leaves can be viewed as an indicator for conditions that may have favored infection of fruit, but the leaves themselves do not contribute directly to the development of black rot on fruit. Black rot infections in fruit may remain quiescent until fruit ripen because green fruit contain inhibitors that prevent fungal growth.
Most fungicides control frog-eye leaf spot, but the SI fungicides (Rubigan, Nova, Procure) and the 3 lb/A rates of mancozeb or Polyram are less effective than captan, Flint, and Sovran. Severity of leaf spotting around fruitlet mummies may be affected by the fungicide program that was used the previous season because fungicides used after thinning may prevent the fruitlets from becoming infected as they dry out during summer. However, the relationship between spray programs, colonization of retained fruitlets by B. obtusa, and inoculum levels within trees has not been documented for most of the fungicides currently available.
Rust-induced leaf spots develop when cedar apple rust and hawthorn rust infections are killed either by subsequent application of SI fungicides (Fig. 3) or by host incompatibility reactions (Fig. 4)
Fig.
3: Rust-induced leaf spot on Ginger Gold leaves where an SI fungicide
was applied to incubating rust lesions. Note chlorotic lesions (black
arrows) with pinpoint necrosis in centers where rust infections have not
yet been invaded by other pathogens.
Fig.
4: Rust-induced leaf spot on an unsprayed rust-resistant selection from
the Geneva breeding program. Severely affected leaves are turning yellow
and will drop during June.
SI fungicides applied within 96 hr of the start of wetting periods will eliminate rust infections before they can cause visible damage to leaves. However, if SI fungicides are applied more than 4 days after infection, leaf cells invaded by the rust fungi will die even though the rust fungus is eradicated. These killed leaf cells result in small 1-2-mm diameter leaf spots that are tan or brown, sometimes with a tiny orange rust fleck in the center of the leaf spot. Similar lesions can appear on McIntosh, Empire, Liberty, and other rust-resistant cultivars if trees are subjected to high levels of rust inoculum in the absence of fungicide protection. On the rust-resistant cultivars, fungal development is arrested by the genetic resistance of the host rather than by fungicide activity, but the resulting leaf spots are similar.
Leaf cells killed by the initial phases of rust infections provide an entry point for other less-pathogenic leaf spot pathogens such as Botryosphaeria, Alternaria, or Phomopsis species. These fungi invade cells killed or damaged by failed rust infections and then move into adjacent healthy tissue, thereby enlarging the leaf spots until the individual lesions look like frog-eye leaf spots. Rust-induced leaf spots can be distinguished from frog-eye leaf spots because the former are uniformly distributed throughout tree canopies, whereas the latter are clustered near inoculum sources. Sometimes the original orange-yellow rust lesion remains visible in the center of rust induced leaf spots, whereas frog-eye leaf spots never have such bright orange centers.
Other leaf spots resulting from fungus-fungicide interactions can develop when SI fungicides, strobilurin fungicides (Sovran, Flint, Pristine), or Topsin M are applied to leaves that contain incubating apple scab or mildew lesions. Scab spots that are arrested during the early part of the incubation period (roughly 5 to 8 days after infection) can produce "ghost lesions." (Fig. 5)
Fig. 5: 'Ghost lesions' of apple scab on a Jerseymac leaf that
received at post-infection application of a strobilurin fungicide.
Ghost lesions are indistinct pale spots 2-3 mm in diameter that develop where the scab fungus disrupted normal cell functions before the fungus was inactivated by the fungicide. The same fungicides applied just before scab lesions become visible can result in rusty, red-brown lesions that exhibit the usual size and shape of normal scab spots. (Fig. 6)
Fig.
6: Fungicide-arrested lesion of apple scab on a Jerseymac leaf that
received an SI spray 13 days after infection.
Post-infection application of the SIs and strobilurins can also cause "burned out" mildew lesions on leaves. Mildew lesions arrested by fungicides can appear on the upper leaf surface as large chlorotic lesions with indistinct margins, or on the lower leaf surface as more sharply-defined red blotches. (Fig. 7). Portions of the leaf compromised by mildew may be more susceptible to subsequent invasion by secondary pathogens that may cause necrotic spots or larger irregular areas of leaf necrosis.
Fig.
7: Fungicide-arrested mildew lesion on an apple leaf.
Alternaria leaf spot appears as brown spots similar in size to frog-eye leaf spots. Alternaria species can be isolated from leaf spots in many orchards, especially in late summer, but Alternaria leaf spot does not cause economic damage in the northeast. In most cases, Alternaria is a secondary invader of damaged leaf tissue. In North Carolina and Virginia, however, a severe form of leaf spotting known as Alternaria blotch spreads rapidly during summer and causes premature defoliation of affected trees. Delicious is particularly susceptible. The strain of Alternaria mali that causes defoliation in the southeast may be different from the common Alternaria mali present in northeastern orchards. None of our fungicides are very effective for preventing Alternaria leaf spot or Alternaria blotch."
Part II - spots caused by pesticide phytotoxicity:
Captan is a potent fungicide on leaf surfaces, but captan is phytotoxic when it moves inside leaves or fruit. Most growers know that captan, if applied shortly before or after an oil spray, can cause severe leaf spotting, especially on Delicious. There is no set delay that can be used for separating captan sprays and oil sprays because leaf condition at the time of application, rates of the two products, and varietal susceptibility to captan make a simple answer impossible. Captan-oil leaf spotting occurs because oil acts as an emulsifier that enables captan to diffuse into leaf cells. Even in the absence of oil, captan penetrates leaves more easily when leaves have developed under extended periods of cloudy, cool weather, because sunlight and dry conditions are required to stimulate development of the cuticle layer that prevents captan from reaching leaf cells. As might be expected, leaf spotting caused by captan-oil interactions is also more severe and the period of susceptibility is more extended when cloudy weather has limited cuticle development.
Captan-related leaf spotting can also occur when captan is tank-mixed with other products that are formulated with special wetting agents or penetrants. The captan label specifically states "The use of spreaders that cause excessive wetting is not advised."
Captan almost always causes some leaf spotting and/or shot-holing on captan-sensitive cultivars of sweet cherry and plum. The severity of the injury varies with the prior weather conditions and resulting leaf condition at the time of application. Leaf injury can be especially severe if captan is applied following cloudy, cool weather during a period of rapid shoot growth.
Over the past 20 years, I have seen cases of leaf spotting that have been traced to applications of various other pesticides, including Sevin XLR, Guthion, Lorsban, and Asana. In some cases, these products had been applied in mixtures with captan, whereas other cases involved mixtures with other pesticides. Most of these incidents did not result in serious leaf damage, and they are cited here only to illustrate that many different pesticides may cause phytotoxic leaf spotting under certain conditions.
In some cases, unusual sequences of pesticide combinations may contribute to phytotoxicity. Last week I visited an orchard with rather severe leaf spotting on mature Red Delicious trees where a tank-mix of Azinphos-methyl plus urea was applied in mid-May and was followed four days later with an application of Agrimek plus 1 gal of summer oil per acre. Adjacent Rome and Spartan trees showed very little injury, and no injury was evident in other orchard blocks that received the first spray of Azinphos-methyl plus urea but not the follow-up spray of Agrimek plus oil. I suspect that the urea softened the leaves enough to allow increased uptake of oil or of oil plus Azinphos-methyl residues when the second spray was applied 4 days after the first spray. Cool, cloudy conditions throughout mid-May was also a contributing factor.
As noted on the product label, Sovran can cause leaf spotting on some sweet cherry cultivars. I have seen this damage on several farms where cherries were growing adjacent to apple trees that had been sprayed with Sovran.
The strobilurin fungicide azoxystrobin (Abound, Quadris, Heritage) is extremely phytotoxic to McIntosh, Gala, and some other apple cultivars. Drift from azoxystrobin applied to other crops can cause a leaf spotting on McIntosh that is indistinguishable from frog-eye leaf spot. Higher concentrations (as may result from residues left in a sprayer when switching from one crop to another) will cause extensive necrosis of leaf tissue and browning or russetting of the skin on apple fruit. The large number of labeled uses for azoxystrobin raises the probability that apple growers in the northeast will experience occasional problems due to off-site drift of azoxystrobin. Azoxystrobin injury should be easy to diagnose because the leaf spotting will appear suddenly, will be evenly distributed throughout the canopy, and will occur only on McIntosh, Gala, and other Mac-related cultivars, whereas adjacent cultivars will be completely unaffected. The varietal susceptibility of apples to azoxystrobin injury is a useful distinguishing characteristic, because no other pesticide or fungal pathogen that might cause leaf spotting on apples would be similarly delimited by cultivar.
Gramoxone herbicide drifting onto apple leaves can cause a brilliant yellow leaf spot, although the spots eventually turn brown and necrotic. Injury from herbicide drift is often more prevalent on low branches, but small spray droplets can drift throughout a tree canopy, sometimes causing an even distribution of leaf spotting that one might not associate with herbicide drift. The potential for foliage injury with gramoxone can be reduced by mixing a drift inhibitor with the herbicide. Drift inhibitors reduce the production of small spray droplets that are easily carried into the tree canopy by even the slightest breeze.
Summary: In commercial orchards that receive timely fungicide applications, most early season leaf spots are attributable to injury from agrichemical sprays. Risks of encountering phytotoxicity on leaves can usually be reduced by using proper sprayer calibration, following label restrictions on pesticide mixtures, and by keeping spray mixtures as simple as possible. The latter includes avoidance of untested mixtures of pesticides, micronutrients, and plant growth regulators, and avoidance of spray adjuvants not specifically required by either pesticide labels or unique water quality or other application conditions. Special care is required in years when the spring growth flush after bloom coincides with an extended period of cloudy, cool weather, because leaves that develop under those conditions are especially susceptible to injury by pesticide applications."
Glen Koehler
Pest Management Office
491 College Avenue
Orono, ME 04473
Voice: 207-581-3882
Email:
gkoehler@umext.maine.edu
Web:
PRONewEngland.org
Fax: 207-581-3881
What we call the secret of happiness is no more a secret than our willingness to choose life. - Leo Buscaglia