Apple Update: May 30, 2017

Crop Stages

• In Southwestern Ontario apples are at 3 to 12 mm fruitlet size
• In Grey County and east of Toronto, apple blossoms are at full bloom to petal fall

Production

Fruitlet thinning applications have begun in Southwestern Ontario and early applications of NAA or 6-BA have started in east of Toronto and in Grey County. The weather is presenting tricky conditions for thinning applications with cool days in some areas. It is better to wait for the right temperature conditions to apply fruitlet thinners rather than waiting for the perfect fruitlet size. For NAA and carbaryl, optimal temperatures are between 21 and 24˚C. NAA should not be applied below 16 ˚C or above 27˚C. MaxCel or Cilis Plus should be applied in temperatures above 20 ˚C.

If you are curious to try the Fruitlet Growth Model, Michigan State University has the model in an excel sheet with the instructions on their apple page http://msue.anr.msu.edu/topic/apples/horticulture . This model helps you to predict which fruitlets will abscise by day 7.  With cooler temperatures than normal, using this model will give you a larger window to decide whether to apply a 2^nd or 3^rd thinner. This model instructs you to mark 15 clusters on 5 trees in a block and measure the size with a caliper on 3 and 7 days after your thinning application. It can be a tedious task and it is best to have two people dedicated to measuring fruitlets during the thinning window. It is best to have one person measuring the fruitlets while one person records the measurements. With my first experience with the Fruitlet Growth Model it takes 4 hours per block to number the individual fruitlets and measure the fruitlets with only one person.

Growers in Eastern Ontario and Georgian Bay will be applying their 1^st applications of Apogee while growers in Southwestern Ontario will be thinking about applying their 2^nd application which usually occurs 2 weeks after the first application.

Disease

Warm temperatures and frequent rains have made for ideal conditions in orchards still in bloom for fire blight infection. For most regions, the fire blight prediction maps are predicting high risk of infection for at least the early part of the week.

While bloom has passed in many regions of the province, growers should be aware of blossoms still open, particularly young trees or rat-tail bloom. Protecting these open blossoms from infection will be critical during times of high risk:

• Antibiotics will break down quickly in sunlight, likely only providing 24-72 hours of protection before a re-application would be required.
• This means any antibiotics applied last week during the main bloom period will have already degraded away.
• If time and labour are available, it may be best to go through the orchard every couple of days and remove the rat-tail blossoms by hand.
• Be sure to remove blossoms in dry weather to prevent the spread of infection.

Some areas have experienced extreme weather in recent weeks including heavy rain, hail and high winds. These events can cause trauma blight. If there is concern, Cueva and/or Double Nickel can protect against spread of infection if used immediately following the trauma event.

Most regions in the province have reached the end of the primary apple scab period. Although all ascospores are now mature, several good rain events are needed for the remaining spores to be released. It’s best to err on the side of caution and keep good fungicide coverage for a few more weeks to be sure. Coverage will be needed anyways during warm and humid weather for powdery mildew control. There have been numerous periods of ideal mildew infection conditions this spring and risk will continue, particularly in susceptible blocks until terminal bud set. Symptoms for both apple scab and powdery mildew have been found. Scab infection remains light in only some problem blocks.

The saturated soils this spring have created ideal conditions for Phytophthora crown and root rot (Figure 1). Symptoms have been found this year on young trees in heavy soil. However, it is also possible infected trees may decline slowly over several years. Typically, purplish cankers can be found at the base of the tree at and/or below the soil line. Tissue necrosis under the bark is often reddish-orange with distinct margins. For more information, see Monitor for Phytophthora root, Collar and Crown Rot Diseases in Fruit Trees in the May 17, 2017 Hort Matters issue.

Figure 1a. Dark, cracked Phytophthora canker above the soil level.

Figure 1b. Phytophthora can appear as reddish-orange canker with a distinct margin.

Anthracnose (Figure 2) and nectria cankers (Figure 3) continue to show up in orchards around the province. These cankers are common a couple of years following a cold winter such as what we experienced in 2014 and 2015. The opportunistic pathogens can enter wounds caused by winter injury, but also mechanical damage, insect feeding or hail. In most cases, a healthy tree is able to defend against the canker. For more information, see Cankers in Apple Orchards in the May 17, 2017 Hort Matters issue.

Figure 2. Anthracnose canker begins to crack as it matures exposing long fibers which give a “fiddle string” appearance.

Figure 3. Nectria canker can look similar to fire blight but produces bright orange fruiting bodies.

Insect

Petal fall insecticide applications continue. Some growers are concerned with rainfastness and the extent of rainfall in many areas. The following chart by John Wise, Michigan State University summarizes the general rainfast characteristics for common insecticide classes. To read the full article, see Rainfast Characteristics of Insecticides on Fruit.

The spray window for oriental fruit moth will be over in most early regions in another week. Some growers have selected insecticides with good efficacy on mullein bug, spring feeding caterpillars, European apple sawfly and/or apple leafcurling midge if monitoring had indicated they had reached threshold or had high pest pressure.

In other monitoring traps:

• Codling moth catch has been increasing.  However, spray timing for 1^st generation egg hatch is still more than 10 days in some regions.
• Obliquebanded leafroller traps are up with some catch reported.
• A significant number of redbanded leafroller have been caught in OBLR traps as well. Leafroller larvae and pupal cases can still be found in rolled foliage.
• European apple sawfly catch has been relatively low likely due to the cool weather during bloom. None the less, growers in areas where this is a pest of concern are applying a petal fall insecticide that has efficacy on sawfly.
• San Jose scale adult flight has begun in the last week in orchards that are being monitored. Pressure is very high in some blocks with trap counts in the 100’s.
  • Degree day models for San Jose scale using first adult catch as the biofix have started to be accumulated.
  • Early estimates are predicting 1^st generation crawler emergence mid- to late June in the early areas.
  • The cooler nighttime temperatures are delaying development slightly.
  • Last week’s Apple Update: May 24, 2017 suggested some control strategies for managing this 1^st generation.

High numbers of mullein bugs are being tapped out in various orchards across the province, exceeding the threshold of 7-9 nymphs per 25 taps. No damage has been reported yet. Growers seeing numbers above threshold are applying insecticides with activity against this pest particularly on susceptible varieties such as Red Delicious, Spartan, Spy, Empire, Cortland, Gala, Jonagold and Golden Delicious.

With the loss of diazinon, there is some concern around how well the remaining control products will work. It’s recommended to continue monitoring after the insecticide has been applied to determine if a follow-up spray in required. Delegate, Altacor and Exirel do not have efficacy on this pest.

The critical time for damage from mullein bug is generally dime to quarter-sized fruit. Despite high pressure in some orchards last year, very little damage was actually observed. These pests are also very good predators of mites, aphids and leafcurling midge. Some growers are wondering if there was enough other food available in the orchard last year that it kept the mullein bug from feeding on developing fruitlets. Some scouts and growers are already reporting seeing nymphs with red bellies, signalling that they have begun feeding on European red mites. At this point, management would no longer be needed.

Aphid colonies continue to be reported. In some orchard blocks, rosy apple aphid activity has been higher than normal with extensive leaf crinkling/curling. This can be mistaken for leafcurling midge, which is also starting to be found:

• Rosy apple aphid damage generally curls leaves under with colonies found on the underside of the leaf (Figure 4).
• Leafcurling midge causes leaves to roll upward toward the mid-vein with small larvae found inside the curls (Figure 5).

Figure 4. Feeding damage by rosy apple aphid causes leaves to crinkle and curl under.

Figure 5. Feeding damage by leafcurling midge causes leaves to roll upward towards mid-vein.

Woolly apple aphid colonies have started to be found as well (Figure 6). A properly timed Movento application for scale, followed with a subsequent application 10-14 days later should also have activity on woolly apple aphid.

Leafhopper activity is increasing. White apple leafhopper and potato leafhopper are present in many orchards. Despite the cool weather, the appearance of potato leafhopper has been earlier than usual. These were likely blown in on the storms in recent weeks since this pest does not overwinter in Ontario. No sign of hopperburn has been reported yet. However, scouts and growers are monitoring young trees in orchards where potato leafhopper has been observed as damage can stunt growth.

Early miticides, such as Apollo, Agri-Mek or Nealta will be applied over the next couple of weeks in orchards with high mite pressure. Refer to Table 3-9. Activity of Miticides Registered on Apples and/or Pear in Ontario in 2016-2017 Publication 360, Guide to Fruit Production for more information on registered miticides.

• Apollo is primarily an ovidicide, which means it targets the egg stage or newly hatched nymphs (typically no later than 14 days after petal fall). Therefore, populations should be monitored to determine the number of eggs present before Apollo is applied. While this product is no longer being manufactured, growers can still use what supplies are available.
• Agri-Mek should be applied within 21 days of petal fall. Since oil is required, do not apply Agri-Mek within 14 days of captan or sulphur fungicides.
• Nealta is a miticide that needs to be applied as populations build, so an earlier application is ideal. Include an adjuvant to improve performance.