A Flood of Fungal Problems?

Friday, 4 July 2014:  Storms over the past three days have provided >4 inches of rain throughout much of the Hudson Valley, and those rains may trigger major disease-control problems in orchards where early-season disease control was less than 100% effective.  (At least the rains have erased for now the water-stress concerns that I wrote about just 4 days ago!)

Apple scab: Rains on 2 July probably removed most of the fungicide protection from apples, at least in the parts of the lower Hudson Valley that had more than 2 inches of rain on Wednesday.  Orchards where no scab was evident on leaves prior to these rains should not be at any risk of scab through the remainder of the season.  However, orchards that  had a little scab showing on leaves (I’ll call them at-risk orchards) might end up with some fruit scab as a result of the second deluge that began yesterday and continued this morning. Hot weather earlier this week, along with captan residues from the last fungicide application, will have reduced the numbers of viable scab conidia that were available at the start of the recent rain events, but apple scab can “revitalize” quickly in warm damp weather.

Where DMI fungicides are still effective , it might be wise to treat at-risk orchards with Inspire Super plus Captan as soon as possible and hopefully within 72-96 hours from the start of the rain on Thursday.  In the absence of DMI-resistance, Inspire Super should provide 96 hr of post-infection activity, and it will also provide excellent protection against summer diseases. If Inspire Super has already been applied 5 times this year (the maximum number of applications allowed per year), then Indar plus captan would be a viable alternative.

For at-risk orchards where DMIs are no longer effective, the best alternative may be Captan plus Merivon or Captan plus Luna Sensation (except in NY where the latter is not registered). Both Merivon and Luna Sensation should provide at least 48 hours of post-infection activity on leaves, although I’m not certain if anyone knows exactly how much post-infection activity they may have on fruit. Both Merivon and Luna Sensation will also provide excellent protection against summer diseases.

A third option for at-risk orchards would be Captan plus Syllit (with Syllit at 3 pt/A since the highest label rate is required for maximum post-infection activity). That option is only available if Syllit has not yet been used in three applications this season, and the disadvantage of Syllit is that it will not provide control of summer diseases. Furthermore, I am a bit concerned about the potential for fruit injury from Syllit-Captan combinations when they are applied in hot weather, although fruit at this point will be less susceptible to damage than they would have been in early June.

Finally, Flint plus Captan applied within 48 hours from the start of the rain may also prevent fruit scab. However, Flint and other strobilurins have generally been less effective than the combinations mentioned above for arresting scab after inoculum levels are high. Because the hot weather prior to our current rains may have inactivated most of the scab conidia, Flint may be sufficient to protect fruit in this situation, but there is a degree of uncertainty about efficacy of Flint in this situation.

Phytophthora root rot: If rains resulted in orchard flooding for even a few hours, then there is a possibility that Phythophthora could begin root infections in susceptible rootstocks. For apples, the risk of infection is greatest for trees on M.26 since the rest of the common rootstocks are fairly resistant. (MM.106 is quite susceptible, but most trees on this older root stock are now so large as to be at lower risk). Trees are M.26 are at greatest risk in their early years when the trunk diameter is still less than 5 or 6 inches.

Where Phythophthora is a concern, a phosphite fungicide should be included in the next spray.  Phosphites are very effective against Phythophthora and are quickly translocated from leaves to roots.  So far as I know, phosphites can be safely tank-mixed with most other pesticides except for copper-containing fungicides. If a phosphite was applied during June, then trees are probably still protected from that earlier application.

Phosphites can also be used on peaches and other stone fruits where there is a concern about Phythophthora.  Labels for the many available phosphites vary, so check to be certain that the product purchased is labeled for this disease on the crops to which it will be applied.

Cherry leaf spot: Sweet and tart cherries that already had a few leaves in the tops of trees turning yellow due to cherry leaf spot will be at considerable risk for secondary spread of this disease. Failure to control cherry leaf spot will result in premature defoliation that can lead to subsequent winter injury. Apply a fungicide as soon as possible after the rain stops. Most of the registered fungicides (other than captan) should provide at least a day or two of post-infection activity, but repeated use of DMIs for brown rot control may have contributed to DMI-resistant leaf spot in some orchards.  

Where cherry harvest has been completed, or where rains have ruined the crop, Syllit can be applied for leaf spot control. Syllit provides a different chemistry than products used for brown rot control and therefore should still be highly effective in most Hudson Valley orchards. However, Syllit will not control brown rot. Cracked fruit that are not protected with fungicides will mummify on the tree and provide inoculum for brown rot in the future. Note that Syllit has a 7-day PHI, so it cannot be used at this point in the season unless cherry harvest has been completed.

Fabraea leaf spot on pears (and quince): Reports I’ve heard suggest that this has been a tough year to control Fabraea on pears. The current rain patterns will further complicate matters for orchards where Fabraea was already evident. I don’t know if any of our fungicides have much post-infection activity against Fabraea, but Syllit is probably the best bet for the next spray if Syllit has not yet been applied three times this season. Flint and other strobilurin-containing fungicides provide some protection, but they have generally been less effective than Syllit.

Where Fabraea infections are already visible on leaves or fruit, one percent spray oil should be added to all fungicide applications through the remainder of the season. Research at the Hudson Valley Lab has shown that oil does not prevent infection, but it  significantly reduces the number of viable conidia that are produced in lesions. Including spray oil in every application after the first Fabraea lesions appear can slow the spread of the disease and help prevent premature defoliation. Repeated applications of oil may cause some lenticel enlargement on young shoots, along with other indications that the trees are not really happy to be repeatedly coated with oil. Because repeated oil applications cause some phytotoxicity and because oil does not prevent infection, oil is not recommended until the first lesions become apparent. Nevertheless, a bit of phytotoxicity from oil is far better than having trees with no leaves at the end of July. Remember that when using spray oil, the oil should be applied at 1% of whatever spray volume is being applied: oil should not be concentrated beyond 1% in the tank.

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