Written by Achour Amiri, WSU, May 2019
The unusual cool and wet spring observed, so far this season in Central Washington, has to be taken into consideration when managing diseases in the orchards including typically orchard diseases such as powdery mildew, but also to minimize fruit infections by pathogens, which may cause rots later in storage.
Powdery mildew of apple
There is a high mildew pressure observed currently in Central Washington especially on susceptible and highly susceptible cultivars such as Honeycrisp, Crisp Pink, Granny Smith, and Golden Delicious. Some mildew pressure has also been seen in unsprayed orchards planted to Gala and even Fuji. This is not due to the recent multiple rain events but rather to the combination of the emergences of new shoots, which are the most susceptible organs to mildew, with moderate temperatures ranging between 60°F to 70°F known to be optimal for mildew infections. The growth of new shoots is expected to end soon, however, there is a high risk that many shoots have been infected and will likely express mildew symptoms in few weeks even if temperatures rise. Although management in susceptible cultivars is critical and may need to be extended longer than usual, scouting orchards planted to moderately-resistant cultivars is highly recommended to detect any infection niches. If the latter are sporadic, a careful pruning and elimination of infected shoots should help lower the pressure.
Most sprays to control mildew should have already been conducted in orchards at this period of the year, but if the second and third cover sprays are not yet conducted, choosing the most effective fungicide is the way to go. A list of most common fungicides registered for mildew control is provided in Table 1. The DMIs should be preferred over fungicides from Groups 7 and 11 in early season to limit selection of resistant populations in other fungi such as gray mold (Botrytis cinerea) and other preharvest and storage rots against which Group 7 fungicides have a better activity than the DMIs. Fungicides such as Procure and Luna Sensations are highly effective against powdery mildew and should be favored if needed. Another relatively low risk fungicide to consider for rotation with fungicides from Groups 3 and 7 is Polyoxin D (OSO) which showed a good efficacy under moderate disease pressure.
Table 1. Fungicides registered to control powdery mildew on apple.
| Fungicide | Active Ingredient | FRAC group1 | Resistance Risk2 | Rate/acre3 |
| Procure | Triflumizole | 3 | Medium | 8 – 16 fl oz |
| Rally | Myclobutanil | 3 | Medium | 5 oz |
| Topguard | Flutriafol | 3 | Medium | 8 – 12 fl oz |
| Inspire Super | Difenoconazole + cyprodininil | 3 + 9 | Medium | 12 fl oz |
| Adament | Tebuconazole + trifloxystrobin | 3 + 11 | Medium | 4 – 5 oz |
| Fontelis | Penthiopyrad | 7 | Medium to high | 14 – 20 fl oz |
| Aprovia | Benzovindiflupyr | 7 | Medium to high | 5.5 – 7 fl oz |
| Luna Tranquility | Fluopyram + pyrimethanil | 7 + 9 | Medium to high | 11.2 – 16 fl oz |
| Luna Sensation | Fluopyram + trifloxystrobin | 7 + 11 | Medium to high | 5 – 5.8 fl oz |
| Merivon | Fluxapyroxad + pyraclostrobin | 7 + 11 | Medium to high | 4 – 5.5 fl oz |
| Pristine | Boscalid + pyraclostrobin | 7 + 11 | Medium to high | 14.5 – 18.5 oz |
| Sovran | Kresoxim-methyl | 11 | High | 4 – 6.4 oz |
| Flint | Trifloxystrobin | 11 | High | 2 – 2.5 oz |
| OSO TM | Polyoxin D zinc salt | 19 | Medium | 3.7 – 13 fl oz |
| Lime Sulfur | Calcium polysulfide | M2 | Low | See label |
| Sulfur, dry flowable | Sulfur | M2 | Low | See label |
1 Same number indicate fungicide from the same chemical group (same mode of action). Rotate with fungicides from different FRAC groups.
2 Risk estimated based on the specific mode of action and maximum number of applications indicated on the label.
3 Check the label for rates and specific timing of application.
Fruit may be at higher risk for infections by pathogens that cause storage rots
Most fungi require optimal temperatures and wetness to increase their inoculum size and possibly infect more fruit. The usual dry weather of Central Washington is what keeps disease pressure low in the orchard to the point that summer diseases are not a concern in the region. However, the numerous rain events seen recently may result in increased infection rate and thus more rots in storage. Pathogens, when present in the orchard, may infect fruit on the tree and if conditions are not conducive it will survive in a form of dormant infections to only develop rot when conditions become favorable. Our recent studies showed that the inoculum size increases from bloom to fruitlet (1-inch diameter fruit) and if no spray is conducted, the inoculum load will increase throughout the season until harvest. The most critical orchard pathogens to be concerned about are Botrytis cinerea (that causes gray mold in storage), Neofabraea species (bull’s eye rot), Phacidiopycnis species of apple (speck rot) and pear (Phacidiopycnis rot). All these pathogens are known to be exacerbated by rain and increased wetness. They particularly infect the calyxes and stem-bowls where any opening will allow them to penetrate the flesh and rot the fruit. Keep a close eye on the health of the tree and make sure cankers are pruned or treated because spores of Neofabraea and Phacidiopycnis can be splashed by rain to infect fruit.
In our recent studies, we found that conducting a spray in the summer (on immature fruit in July) in addition to one at fruitlet stage and one before harvest significantly reduced the inoculum size at harvest compared to just a standard 7-day preharvest spray. This may be very relevant this year especially if more rainy events occur in the next weeks or months.
Materials listed above in Table 1 have variable efficacies against rot pathogens. Fungicides from Group FRAC 3 are more effective against mildew, whereas FRAC 7, 9 and 11 fungicides are good against mildew and rot-pathogens as long as resistance to them has not already developed. OSOTM (FRAC 19) has good efficacy against Botrytis but its efficacy against the other pathogens needs further assessment. Sulfur or lime sulfur have marginal efficacy against rot-pathogens. Refer to Table 2 below for more information about specific efficacy against each pathogen. Materials listed in Table 2 include those with only preharvest applications and those with only post-harvest applications.
Table 2. Efficacy of most common fungicides against three major pathogens infecting apple and pear preharvest
*: Efficacy assumed that the fungus has not developed resistance to a given fungicide.
**: Some Resistance of Botrytis to Pristine has been documented in Central Washington.
***: Academy is available for drench application so far.
+, ++, +++, ++++ indicate low, fair, good and very good efficacy. UN indicates unknown and? indicates that further tests are need.
Contact
Achour Amiri
WSU Tree Fruit Research and Extension Center (TFREC)
Tel: 509-663-8181 ext 268 (office) / 228 (Lab)
Fax: 509-662-8714
E-mail: a.amiri@wsu.edu
Disclaimer
No endorsement is intended for products mentioned, nor is lack of endorsement meant for products not mentioned. The author and Washington State University assume no liability resulting from the use of pesticide applications detailed in this report.
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