Download the recording of the July 19, 2011 call here.
FIRST: Some important notes from John regarding CM
-Â Â Â Â Â Â Â Â Â GROWERS: Please calculate your degree days from your first biofix and send them to John (email@example.com) AS SOON AS POSSIBLE.
-Â Â Â Â Â Â Â Â Â Further, Iâ€™d add two things:
- One: If youâ€™re at 900 degree days from your first biofix then you should start seeing some of your second flight.
- Two: This heat wonâ€™t necessarily accelerate CM development. There is an upper temperature limit on the CM model. If itâ€™s above 86 or 88 degrees, it wonâ€™t speed things along. The heat actually might cause some mortality.
-Â Â Â Â Â Â Â Â Â Finally, make sure your CM traps are up-to-date and keep me informed.
-Â Â Â Â Â Â Â Â Â Right now, weather is the dominant factor. Temperatures everywhere are reaching the mid-90s with high humidity and hot nights. This weather is going to be dominating a lot of our activity.
Questions with Dave Rosenberger
-Â Â Â Â Â Â Â Â Â 2:45 â€“ John: Does conidia production slow down or shut down in the heat?
- Dave: As in all biological systems, the answer is never just â€œYesâ€ or â€œNo.â€ The best work on this was done in Pennsylvania by Dr. Fred Lewis to 30 years ago. They looked at conidial production and viability over a period of time. What they found was that once you get a couple of days of temperatures above the mid-80s, the viability of the conidia drops off. So even though conidia are being produced, they arenâ€™t very viable, and that lack of viability then reduces the likelihood that scab is going to spread as rapidly as it would under cooler temperatures.
- However, that doesnâ€™t mean that it canâ€™t spread. Certainly, as you pointed out, those lesions will constantly create new conidia. So if the heat disappears and you get a week of temperatures in the 70s with rain, there is still a risk of scab spreading.
- Generally, I feel that July and August are time periods when scab is relatively benign, even if you have scab in the orchard. This for several reasons:
- The heat. Foliar scab only affects newly unfolding leaves. So if you have terminal bud set, there is really no tissue there anymore that is susceptible during mid-Summer. And the fruit, once they are about 1â€ in diameter, become more and more resistant. It doesnâ€™t mean they canâ€™t become infected, but you need a longer wetting period (so the Mills period really doesnâ€™t apply anymore). Prior to harvest, youâ€™d need at least a 24-hour wetting in order to get a significant amount of fruit infection, and probably 48 hours to really get an outbreak of storage scab or pinpoint scab.
- Now, things change once you get to the latter part of August because the leaves then become susceptible to under-leaf infection. You can get a lot of scab buildup on the undersides of leaves. When that happens, if you do get a wet harvest season, the conidia from the undersides of the leaves will be on the fruit or have caused infections prior to harvest and result in a lot of pinpoint scab. Thatâ€™s not an issue if youâ€™re selling the fruit immediately off the field, but if youâ€™re storing them for 60 days or more, you could end up with scab showing up during storage.
- Lesson: All this is sort of a long way of avoiding answering your question. I think when you have scab you canâ€™t turn your back on it even in July and August, but I would say that a modest rate of Captan applied every 2 weeks to 18 days is probably enough to keep it in check. Then, the biggest adjustment you may need to make is applying one additional spray pre-harvest compared to what you might do in a year when there is no scab in the orchard.
-Â Â Â Â Â Â Â Â Â 7:42 â€“ John: To clarify something that Dr. Rosenberger said: when those leaves become susceptible to scab late in August, thatâ€™s a result of senescence. Is that correct?
- Dave: Yeah. I certainly donâ€™t understand all the biochemistry involved in that. But for whatever reason, the underside of the leaf becomes susceptible. The scab on the underside doesnâ€™t look at all like normal scab lesions. Someone not attuned to looking for that might overlook it and say itâ€™s just some browning due to mites or some other obscure cause. The lesions on the underside can be very diffuse â€“ not nearly the discrete kinds of lesions you see earlier in the season on the upper surface.
- If you allow that under-leaf buildup, it creates potential for a lot more ascospores the following year. Thatâ€™s another reason to be a little bit concerned about what happens in late-August and September. Again, if itâ€™s a fairly warm and dry harvest season, that wonâ€™t be a big issue. But if you get a cool, wet harvest season, that ascospore buildup becomes moreÂ of a concern.
-Â Â Â Â Â Â Â Â Â 9:20 â€“ John: Itâ€™s one thing to ask growers in the end of August, when theyâ€™re already into harvesting the early varieties, to go out and apply a high rate of Captan to Macintosh and Cortland. But basically youâ€™re saying that the grower should spray Captan if those apples are going to be stored for a long period of time and the orchard had significant scab earlier in the season?
- Dave: Yeah, but the issue with high rates of Captan, at least for growers in the area Iâ€™m working in, is all the visible residue. Thatâ€™s where things get a bit complicated.
- I noted in the notes you sent me that you expressed concern in terms of resistance management about using strobilurin fungicides during the summer. I think thatâ€™s certainly valid, but when you think â€œWhat are we going to control scab with other than Captan?â€ The obvious choices would be Flint (with a 2-week pre-harvest window) or Pristine. The advantage of those is that they donâ€™t leave a visible residue. So even though it may produce selection pressure for resistance, weâ€™ve been using strobilurins in that pre-harvest spray just to try to get away from all of the visible residue. If I were an apple picker, Iâ€™d hate picking apples treated with a full does of Captan shortly before harvest.
-Â Â Â Â Â Â Â Â Â 11:20 â€“ John: Thatâ€™s a good point. Iâ€™m scared to death of losing our strobilurins, too. But if a grower was willing to apply a spot spray of Pristine on the most scabby cultivars (which has a combination of two classes of fungicides, both of which do have efficacy on scab), they might actually reduce the amount of potential overwintering scab?
- Dave: Well, it would at least keep that underleaf scab from building up.
- In fact, Pristineâ€™s non-strobilurin component really doesnâ€™t have much activity on scab, so weâ€™re still applying a lot of selection pressure for scab resistance.
-Â Â Â Â Â Â Â Â Â 12:18 â€“ John: George Sundin mentioned this last January that Boscalid, the non-strobilurin component of Pristine, does have activity against scab, at least as a protectant.
- Dave: Well, I ran the two components separately in a field trial back when Pristine was being introduced, and I found very little benefit from the Boscalid. But Iâ€™d have to go back and see whether some of those sprays depended on post-infection activity
- So it may have some protectant activity, but Iâ€™d say itâ€™s pretty weak against scab.
- However, George Sundin may have more recent evidence to suggest the contrary. Iâ€™ll have to check with him.
- I think that when weâ€™re applying Pristine, weâ€™re still putting significant selection pressure on the strobilurins.
- Nevertheless, from a practical point of view, and also for summer disease control, weâ€™ve been using a fair bit of either Flint or Pristine late in the season to get away from Captanâ€™s visible residue issue.
-Â Â Â Â Â Â Â Â Â 13:48 â€“ John: Here, weâ€™ve been trying to avoid using strobilurins or the sterile inhibitor Endar. Weâ€™ve been doing that for a number of years with the hope of retaining our ability to use strobilurins. However, if you tank-mixed a lower rate (half-rate or lower) of Captan with Flint or Pristine to avoid that heavy residue at harvest, perhaps you would not put as much selection pressure on the strobilurins?
- Dave: Yeah, I would agree that thatâ€™s the ideal approach: to keep Captan in the mix. The other reason for doing that is that Captan is still our best protection against Bitter rot. So, while Flint and Pristine have some activity, if you have a Bitter rot concern, you really need to keep at least a half rate of Captan in that mix. It serves multiple purposes. By going to a half-rate you get rid of some of the residue. Again, if youâ€™re mixing with Flint and youâ€™re two weeks away from harvest, hopefully youâ€™ll have at least a little bit of rainfall to dilute some of that off before harvest.
- The advantage of Flint is the cost. I know growers here flinch at the cost of Pristine.
-Â Â Â Â Â Â Â Â Â 15:53 â€“ John: Over the last 6 or 7 years weâ€™ve had increasing amounts of Fruit rot in our orchards in late August to early September. Most of it has been Black rot, but last year we had a significant amount of bitter rot. The Black rot that causes fruit rotting is the same thing that causes Frog-eye leaf spot and cankers. When does Black rot spread, and where do Black rot and Bitter rot overwinter?
- John: One thing weâ€™ve already learned: Captan is an effective material against Bitter rot.
- Dave: A lot of work still needs to be done on these pathogens in northern climates.
- Bitter rot and Black rot are very different.
- Bitter rot:
- Bitter rot primarily moves in during late July and August. Itâ€™s been observed that it moves in primarily on the sun-exposed sides of fruit, although not exclusively.
- Iâ€™ve also observed that it moves in on fruit that has been sunburned or heat-injured. When we get high sun exposure on a hot day (and especially on a hot day combined with high humidity), we get a little bit of injury that might not even become visible if it werenâ€™t invaded by rot.
- Itâ€™s very difficult to protect from this kind of invasion following injury by sunburn or heat.
- Once it gets started, it will spread to other fruit in the tree.
- Captan is our best defense. Flint and Pristine give decent control. Sovereign in high rates does the same as Flint.
- In North Carolina, where they have severe Bitter rot pressure, they end up putting on a combination of full rates of Captan plus Ziram. We havenâ€™t found that necessary in New York, but in NC they just canâ€™t get enough material on the trees in their hot humid conditions in order to slow Bitter rot down.
- Overwintering: Once Bitter rot gets established in an orchard, I think it does overwinter on fallen fruit. But frankly, we donâ€™t know all the ways it overwinters. It does have a lot of wild hosts. In our area, forest chestnut develops an anthracnose caused by this same Bitter rot fungus where the leaves turn brown and fall off this time of year. There is a leaf spot on poplar, aspen, cottonwoods, etc. that is caused by the same fungus. In a few cases, growers have had some luck by removing some of these hedgerow trees when they fought Bitter rot for two or three years and couldnâ€™t get it under control. At the same time, though, they jacked up the rates of Captan for a year and cleaned it up that way.
- I canâ€™t comment specifically on which factors come into play, but I do know that once it gets established, it can be difficult to get back under control.
- We rarely see it until after weâ€™ve had super-hot, super-humid weather (90 degrees + 95% humidity) and fruit that are getting to the size that we commonly have in mid- to late-July and then on into August.
- John: I think that pretty much agrees with our experience. The first time I saw it was two years ago in 2009 when we had a lot of sunburned fruit. A lot of those damaged areas of those fruits became infected. The scariest thing for me about Bitter rot is the ease with which it seems to spread from fruit to fruit. You see clusters of un-thinned fruit that are decimated by the Bitter rot.
- Dave: There is just so much inoculum produced on a lesion on a fruit that anything thatâ€™s close to that gets clobbered with inoculum.
- Itâ€™s worth noting that green fruit have some natural inhibitors that reduce their susceptibility to both Black rot and Bitter rot. As the fruit ripens, particularly with Black rot, those inhibitors disappear.
- With Black rot in particular, you can have latent infections in the lenticels and then the Black rot suddenly appears as the fruit matures, because the natural inhibitors are disappearing.
- With Bitter rot, Iâ€™m not so sure about the latent infections. I think the natural defense of the apple tends to prevent infection. But if thatâ€™s compromised by heat injury or by injury from some other chemical, then it allows Bitter rot to get established and you have to then begin to fight a huge amount of inoculums.
- 24:32 – On calcium sprays as a possible contributor to Bitter rot spread: Iâ€™ve been concerned that calcium salts combined with hot weather may actually create or increase stress on the fruit and open the fruit up to more Bitter rot injury, either due to heat, water stress, or something else.
- We just sent out a notice to our local growers suggesting that they not put on calcium sprays in the upcoming high-90s weather.
- 25:18 â€“ John: Would it make any difference if the sprays werenâ€™t calcium chloride, but something else like calcium carbonate â€“ the more expensive forms of calcium that donâ€™t have the chloride ion in them?
- Dave: I donâ€™t think anyone has done any research to be able to comment one way or the other.
- Whether the chloride ion is part of the problem or whether itâ€™s just any calcium on the fruit surface, I donâ€™t know. I canâ€™t even say for certain that calcium is involved, but itâ€™s something to consider when weâ€™re looking at Bitter rot (which requires some sort of surface damage to get started).
- 26:25 â€“ John: When is the peak of Black rot spread?
- Dave: You said that you think the peak of Black rot spread might be during the period right after petal fall. Iâ€™m not so sure, though, in our area, at least.
- One of the primary sources of Black rot inoculum for fruit infections are the retained fruitlets (mummies) that were killed by thinners or fail to pollinate but donâ€™t abscise from the tree. Some varieties like Cortland or Honeycrisp tend to retain a lot, and others like Macintosh, very few.
- Infected fruitlet mummies probably became infected after petal fall.
- In general, there are a few of those mummies around. Even if only 4 or 5% of those fruitlets have black rot, they are a potent source of inoculums.
- However, by my observations, those infected mummies rain down spores throughout the summer. When Iâ€™ve done Sooty blotch/Flyspeck trials and have omitted sprays in July, Iâ€™ve run into Black rot problems. The spores get into lenticels and then cause fruit spotting at harvest and in post-harvest incubation.
- Black rot complicates things if youâ€™re trying to minimize sprays for Sooty blotch/Flyspeck. If you extend too far into July, youâ€™ll experience Black rot getting into lenticels and showing up, not in July, but at pre-harvest.
- 28:49 â€“ John: You mentioned a couple things that get my attention, and it makes me feel better about asking growers to put on a Sooty blotch/Flyspeck material last week just because we were half-way through July and despite the leaf wetness hours.
- 29:21 â€“ John: One thing you said is especially important to note: that this seasonâ€™s mummies can spread Black rot infection in the same season. Iâ€™ve always assumed that infections came from last yearâ€™s infections overwintering on mummies.
- Dave: Actually, it can be both.
- As fruit starts to die, it loses all its natural inhibitors. Once the fruit dies, fungicide sprays alone will be able to reduce the number of mummies that become infected.
- However, Kerik Cox at the Geneva Station found that fungicide were less effective than anticipated at this point.
- 30:41 â€“ John: Black rot would be considered a non-obligate pathogen, right?
- Dave: I donâ€™t know about that, whether it survives in completely dead wood or not. I would classify it as a weak pathogen. It isnâ€™t very aggressive like apple scab or brown rot. But any time there is compromised host tissue, it moves in pretty quickly.
- John: It doesnâ€™t behave in the same way as a very strong pathogen, and itâ€™s harder to pin down what conditions it needs.
- 31:43 â€“ John: The reason I mentioned that the Black rot spread in the first weeks after petal fall is that it does correlate with the symptoms of Frog-eye leaf spot. But that might be more tied to the susceptibility of leaves, because we donâ€™t see a lot of Frog-eye later on. What would you say about that?
- Dave: It appears that the leaves are susceptible during long, relatively cool wetting periods. I rarely see leaf infections occurring much after petal fall. For whatever reasons â€“ maybe there is more fungicide residue held in the larger canopy volumes â€“ I usually see the leaf symptoms appearing as a result of relatively long wetting periods that occur from about Tight cluster to Petal fall. At that point, youâ€™ll get a bloom of foci in trees. Those leaf symptoms are often focused below mummies. The overwintering mummies rain down spores, and when thereâ€™s a long enough wetting period and low fungicide residue, they move in.
- The interesting thing is that nobody has ever shown that spores come out of those leaf lesions. I think theyâ€™re sort of a dead end.
- John: Yes, Patty McManus has mentioned that a number of times. The leaf lesions are more important as convenient markers for us as scouts than for their ability to spread infection.
-Â Â Â Â Â Â Â Â Â 34:12 â€“ John: In a study about 10 years ago, Patty McManus and Mark Gleeson at Iowa State tested the summer disease model. They found that, in Wisconsin, the relative humidity hours actually correlated better with Sooty blotch/Flyspeck than did leaf wetness hours. Weâ€™ve continued to use leaf wetness hours, however, because most people have leaf wetness sensors and not relative humidity sensors.
- Dave: What thresholds are you using say itâ€™s time to spray?
- John: Traditionally, weâ€™ve started counting from petal fall, or at the time (or around a week after) you applied a EBDC, Flint, or Sovereign (if applied after petal fall).
- The first time, we count to 250. But that is when symptoms are supposed to appear. Initially we were only counting wetting periods that were 4 hours length. A lot of growers, however, donâ€™t have data loggers that can tell us how long the wetting periods were â€“ we can only get total number of leaf wetness hours in a day.
- In another paper, Dan Cooley compared several different brands of data loggers with the old hydrothermographs. He found that the Spectrum was considerably behind other models in accumulating leaf wetness hours. The others agreed fairly closely with the hydrothermographs (horsehair design), but the Spectrum seemed to underestimate leaf wetness. The point in time when the Spectrum model would hit the threshold was always a week or two weeks later than other indicators.
- Dave: Dan Cooley and I just put together another paper in which we compared what we really know about Sooty blotch/Flyspeck models. The bottom line is that there is a heck of a lot we donâ€™t know.
- There are huge amounts of variabilities: in the organisms that exist in different parts of the country, in the kinds of wetting periods (dew vs. rain), and also in the ability of our instruments to measure leaf wetness or relative humidity appropriately.
- Iâ€™ll explain what I use, but Iâ€™m not convinced itâ€™s the only way to go.
- In New York, weâ€™ve been starting counting at petal fall regardless of what spray is put on. We assume that at least one good summer disease spray like Topsin-M, Flint, or Sovereign is applied at first cover. That spray sort of takes care of the Flyspeck ascospores that are blowing in early.
- We then feel that it takes about 270 hours of wetting on a string recorder (like on an old Dewitt recorder) â€“ and that might be 200 hours on a Spectrum, or a little less â€“ before we get secondary inoculum blowing from hedgerows.
- I would say that you want to look at 200 hours from petal fall on a Spectrum machine before you say: now Iâ€™m getting inundated with inoculum from hedgerows.
- More than 100 wild hosts support these fungi. So if you have any kind of woodland around your orchard, you will have a lot of influx from outside of the orchard.
- At this point, it will require another 270 hours (about 200 hours on a Spectrum) before you see any symptoms. So you could delay a little further, but the problem is that when we apply fungicides after infection has occurred, we donâ€™t ever eradicate all the fungus. We stop the growth while the fungicide is there, but then a certain proportion of colonies can take off and grow again.
- So if you delay into that second set of 200 hours from petal fall â€“ letâ€™s say you donâ€™t apply a spray until 300 hours from petal fall in mid-Summer â€“ that means you probably already have some incubating infections, at least for Flyspeck.
- Then, if you have a wet harvest period and your fungicide residues run out pre-harvest, those flyspeck colonies are going to develop and pop up suddenly during the pre-harvest interval.
- So, my philosophy on controlling summer diseases is that you can play some games in June, especially if itâ€™s a dry June, but by mid-July you need to be cautious â€“ both for Black rot control, and because you donâ€™t want to give Sooty blotch/Flyspeck fungi a head start.
- In addition, we need to recognize that from mid-July through the rest of August, it gets harder and harder to cover the backsides of the fruit with fungicides as the fruit size increases and limbs bow down under the weight of the cropload.
- One of the most important aspects of Sooty blotch/Flyspeck control is getting good coverage.
- If you let things get a head start in July when you still could get good coverage, you are stacking the deck against yourself.
- 42:05 – John: Iâ€™ve always said that we should put something on as soon as we get to 175 hours leaf wetness hours on a Spectrum. One thing that we donâ€™t do here is assume that a Topsin-M or strobilurin went in post-petal fall, though.
- Dave: I should add that Manzate at a half rate would serve the same role. Manzate is very effective on these fungi. For Black rot, though, youâ€™d need a higher rate.
- 43:07 â€“ John: The other factor that might play in here is that those growers who have continued to stay on top of their Captan program against scab â€“ spraying a high rate on a 2-week basis. What effect would that have on these summer disease organisms.
- Dave: I think Captan at a high rate is reasonably effective. Itâ€™s not as good as a Topsin or strobilurin at controlling Sooty blotch/Flyspeck. Importantly, Captan is more susceptible to wash-off. If youâ€™re having a dry summer, that high rate of Captan alone will do fine. But if youâ€™re getting 1.5â€ to 2â€ of rainfall during that interval, youâ€™ll have lost most of your protection against Flyspeck during that interval.
- One thing weâ€™ve found is that you can get the same activity out of a phosphite fungicide combined with Captan as you do with Topsin-M. Iâ€™m not sure if itâ€™s any more cost-effective. But some of the folks exporting apples in New York find that the buyers in England donâ€™t want Topsin-M applied within 45 days to harvest.
- The phosphite-Captan combination does great against Sooty blotch/Flyspeck.
- But it does not add anything for Black rot control. You must make sure to have a decent rate of Captan to control Black rot if you decide to add a phosphite.
- 45:00 â€“ John: Weâ€™ve talked at length about these materials. The first phosphite material (or â€œphosphorus acid fungicidesâ€) was Aliette â€“ now there is Prophyte, Agri-Fos, etc. They are short-lived materials that are extremely systemic. They are not retained very long, although maybe a little longer in the fruit. Is that correct? Can they act as a protectant?
- Dave: In fact they are effective protectants. Nobody understands why putting them with Captan is so effective. But Iâ€™ve run them with Captan and compared them with Pristine at the end of the season.
- Pristine gives, by far, the longest residual activity against rots and summer diseases. So if youâ€™re looking to protect with a spray in late August, Pristine is a Cadillac material.
- But Captan plus a phosphite is almost as good and will last as along as Topsin+Captan.
- Dave: In fact they are effective protectants. Nobody understands why putting them with Captan is so effective. But Iâ€™ve run them with Captan and compared them with Pristine at the end of the season.
- 46:24 â€“ John: For us, these phosphite materials are very expensive if theyâ€™re used at the upper range of recommended rates (5 lbs/acre). Theyâ€™re costing us $11/lb. They all are very similar in terms of their active ingredient.
- Dave: They are all rather expensive, even the liquids. So they may not be a good alternative to Topsin-M unless you either canâ€™t use Topsin-M for marketing purposes, or if you surpass the 64 oz/acre/year limit.
-Â Â Â Â Â Â Â Â Â 48:30 â€“ John: Silverleaf seems to be on the increase. There isnâ€™t much information about how to control it other than recommendations to cut it out and take it out of the orchard. What can you say about silverleaf? Do these phosphites have any efficacy against it?
- Dave: Silverleaf is one of the fungi that I would classify as a xylem-inhabiting basidial mycede (?) or â€œbrack fungus.â€ It gets into pruning wounds, primarily in trees that are compromised by winter injury or some other factor. It can live inside the tree for years without causing symptoms. The trees are constantly trying to wall off these xylem-inhabiting fungi. When the trees are under no stress, they compartmentalize the fungus and prevent the toxin released from the silverleaf fungus from causing symptoms. When the trees are stressed, however, they canâ€™t maintain those defenses and you start seeing symptoms.
- I donâ€™t think thereâ€™s any product that will control it. I donâ€™t know if anybody has tried phosphites.
- I think itâ€™s a resident inhabitant in apple trees.
- In my experience, it shows up about 4-6 years after a severe winter event (either a Fall freeze or a deep Winter freeze) because that injures the root enough that the pruning cuts canâ€™t heal. When it gets into the cuts, it resides in the trees forever after.
- I donâ€™t see any magical cure for reducing symptoms other than maintaining tree health.
-Â Â Â Â Â Â Â Â Â 51:54 â€“ John: If youâ€™re concerned about its spread and want to reduce inoculum, would it be alright in late season to prune out those branches that are severely affected?
- Dave: The fungus spreads from tree to tree from spores that are released from little â€œbrack fungi.â€ Those silverleaf â€œbrack fungiâ€ are only about the size of a dime.
- They generally wonâ€™t sporulate until the wood is fully dead. So a living limb with symptoms of silverleaf doesnâ€™t pose a danger for further spread. But if you prune that out an leave it at the base of the tree, it will rapidly sporulate and send out spores that will enter new pruning cuts.
- John: So if you prune it now because you see it now, it doesnâ€™t necessarily reduce the likelihood within the tree.
- Dave: No, it wonâ€™t. In fact, in some cases â€“ such as in big old Macintosh trees weâ€™ve seen in northern New York â€“ a branch that has symptoms one year may not have symptoms the next year. The tree may be able to get the fungus under control internally, and it may come back symptomless in future years.
- A badly-effected branch that is declining should be removed, however. The trick is not to leave dead limbs around the perimeter of the orchard or under the trees, because they will sporulate and send inoculum up to fresh wounds.
-Â Â Â Â Â Â Â Â Â 52:55 â€“ John: Twenty-five years ago, we saw silverleaf behaving like this in larger root stocks. If itâ€™s in smaller rootstocks like M9 or Bud9, will it behave the same?
- Dave: I donâ€™t know. With the smaller rootstocks, the stress event that may allow the fungus to continue to progress would be drought stress during summer. The dwarfing stocks just donâ€™t have the capacity to survive drought that the older, bigger rootstocks had. If the tree is under drought stress, it canâ€™t maintain barrier zones. I havenâ€™t seen any data on this with silverleaf in particular, but I know from my work with other xylem-inhabiting basidial mycede pathogens, drought stress can be a big factor.
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