Current DD Accumulations

Chemical Thinning Thoughts

Flowers that were not damaged by frost and were subsequently pollinated should have grown to a size where a good assessment of initial set can be made. Fruit set may vary by block, so individual thinning strategies may be required for each block. Fruit are most vulnerable to all thinners when they are 7 to 12 mm in size.

Thinning chemicals. Growers have four chemical thinners available: NAA, NAAm, carbaryl, and Accel. Carbaryl is considered a mild and safe thinner, since it rarely over-thins. For this reason it is frequently the thinner of choice. Where aggressive thinning is required, carbaryl cannot be depended upon to do the whole job. NAA is frequently used where more aggressive thinning is required. Accel when used by itself is generally considered a mild thinner. Where moderate thinning is desired we recommend application of combination sprays containing carbaryl. Carbaryl plus Accel and carbaryl plus NAA have proved to be very good combinations. Where very aggressive thinning is appropriate we recommend the addition of 1 quart of summer oil per 100 gallons of spray with carbaryl and either Accel or NAA (if Captan is not being used as a fungicide).

Timing of thinning sprays. All thinners work best when moderate to warm temperature occurs during the first 3 to 4 days after application. Ideally, we would like to see daytime temperatures in the uppers 60's to the upper 70's during this period of time. In some years it is best to delay thinner application to coincide with a warm period, rather than apply a thinner at specific fruit size timing. This year may be an exception.

Special conditions this season. Orchards in most areas are dry, and now that significant leaf area has emerged, trees could be under some water stress. In general trees that are stressed thin easier. Rain in the past couple of days has alleviated this water stress somewhat, but we are uncertain how much the stress on the trees has been relieved. If you have received over an inch of rain, it is likely that short-term stress has been reduced to the extent that moderate thinning will be required in heavy blocks with heavy fruit set.

Thinning Apogee-treated trees. If you have applied Apogee on trees that now have a moderate or heavy set, it will be necessary to thin these trees more aggressively than you may have done normally in the past. Generally we recommend that you use an additional thinner rather than increase the concentration of the thinner you normally use. For example, if you normally use Accel or carbaryl, apply both. If you normally use Accel and carbaryl, include a quart of summer oil per 100 gallons on Apogee-treated trees.

Weather this coming week. The weather forecast by the National Weather Service for the next 7 days predicts mostly cloudy with temperatures generally in the lower 60's during the day and in the 40's at night. These conditions are only fair for good thinning activity. We are somewhat skeptical about the accuracy of weather forecast longer than 24 hours, but it is the best that we have. However, if this prediction does prove to be correct, a cautious and conservative approach to thinning may be disappointing. A moderate thinning approach is suggested under these weather and orchard circumstances. If this thinning window of opportunity is missed (as mediocre as it may be), it may be very difficult to thin in 7 to 10 days when—although the crop load can be more accurately judged—fruitlet size is larger and are harder to thin. In our estimation, if you have not already started to thin, this week is the right timing to apply thinning sprays, however, weather conditions may prove an impediment.

Peachy Tasks

Peach growers should be looking at two jobs right now. First, make sure you have fungicide coverage during shuck-split and cover sprays to control brown rot. We appear to be moving into a wetter weather pattern finally—perfect conditions for the brown rot fungus to gain a foothold. Of course you should have been covered with a fungicide during bloom, but if not, the shuck-split through cover spray period is also an important control time. Fungicides of choice include: the protectants Bravo (not to be used after shuck split), Captan, and sulfur; the SI's Elite, Indar, and Orbit; and the Benzimidazoles Benlate or Topsin-M. As always, read and follow the label for specific uses and restrictions. Job two is to give those growing peaches a good shot of ground nitrogen fertilizer. Peaches are heavy nitrogen users, which is needed to support all that new growth producing next years crop. So now is the time to keep the wheels turning by supplying additional nitrogen. Peaches need approximately 0.025 to 0.05 pounds actual nitrogen per year of age, so for example five-year-old peach trees should receive 0.125 to 0.25 (one-eighth to one-quarter) pound actual nitrogen annually. This equals one-quarter to one-half pound (4–8 oz.) urea, best applied in a split-application, the first application now, followed by the second half-pound in thirty days. Adjust the nitrogen application rate depending on soil vigor and crop load, and be careful when using urea to spread the fertilizer evenly in the root zone.

Plum Curculio

Since about bloom, the weather has not been all that favorable for either extensive PC immigration into orchards or extensive egglaying. Here are some data on the pattern of PC emigration from overwintering sites in woods at the Horticultural Research Center. We have 96 odor-baited traps set up at edges of woods surrounding a 2-acre block of apple trees. The traps capture immigrating PC before they reach the apple trees.

May 1-3: 106 total PC captured
May 4-7: 74 total PC captured
May 8-10: 41 total PC captured
May 11-14: 76 total PC captured
May 15-17: 6 total PC captured
May 18-21: 38 total PC captured

Here are some data on amount of PC egglaying injury to fruit from examination of apples on perimeter rows of trees in 12 commercial and 2 unmanaged blocks located in various parts of the state.

The combined data indicate that PC immigration has slowed considerably since the initial high burst from May 1-14. It picked up a bit during the sunny and marginally warm days of this past weekend, but will probably come to a halt during the next few days of cooler and wetter weather. PC injury to fruit was very low in both commercial and unmanaged orchards through this past Saturday (May 19), but increased somewhat on Sunday/Monday in unmanaged orchards.

So what does this information tell us? It suggests that growers have done a very good job of protecting apples against PC with sprays applied at or soon after petal fall. It also tells us that we should be on guard to protect against the next wave of immigrants, which we anticipate will occur when the sun returns and afternoon temperatures reach 70o or greater. For growers who have not yet applied any spray against PC, it might pay to examine some fruit on perimeter trees near traditional hot spots. Rainy weather combined with temperatures of 60o or more (especially above 65o) is conducive to PC egglaying.

Leafminers

At the Prokopy orchard in Conway, spotted tentiform adults remain active. Captures on sticky red rectangles placed horizontally in tree canopies were as great this past week as the previous week. So egglaying is probably still very much in progress and could continue for some days to come. Eggs turn into larvae that show up first as sap-feeding miners on lower surfaces of leaves 10-14 days after eggs are laid. Our best estimate is that imidacloprid (the active ingredient of Provado) has enough residual effect (after entering foliage) to kill LM larvae up to 2 weeks or so after application. Applications made at petal fall may not have enough punch left to suppress miners from late-laid eggs.

The earliest developing miners seen in commercial orchards are now in the sap-feeding stage (no tissue feeders yet). Some orchards are showing up to 2-3 miners per fruit cluster, primarily on the 3rd, 4th or 5th oldest leaves. Densities of 7 or more first-generation miners per cluster could be harmful to early fruit growth, but we don't often see this level of injury. Densities of 1 or more first-generation miners per fruit cluster may foretell a second generation population that is close to the economic threshold of 2 miners per leaf.

We do not recommend treating against first-generation miners but instead recommend treating against second-generation adults, eggs or young larvae if first-generation miners exceed acceptable levels.

Mites

The good news is that motile mites are not turning up in orchards that received 2 oil sprays. The less than good news is that they are turning up (though not yet in substantial numbers) in some orchards that received only 1 oil spray. Cool or wet weather is not favorable for rapid buildup of mites. So almost all orchards remain in good shape with mites and should continue to do so as long as the weather doesn't turn hot and dry.

Leafminers

A few mid-size nymphs of white apple leafhoppers have been seen here and there. Not to worry. Thinning sprays of Sevin do a fine job on these nymphs.

Ideal Infection Weather

Well, all things tend to even out over time, weather included. After one of the driest pre-petal fall season's ever, we are entering a prolonged soaking. Just in case you haven't heard a word at the last two twilight meetings and in the last couple of Healthy Fruit newsletters, THIS IS THE BIG ONE!! That is, most if not all of this year's crop of scab ascospores have been shot into the air over the last two days. The soaking rain is keeping the foliage wet, and conditions for infection could not be better (for the pathogen) or worse (for the grower),

Hopefully, orchards were covered with a protective fungicide before the rain started. If not, then prepare for a couple of applications of an SI (Nova, Rubigan, Procure) plus a protectant, or alternatively one of the strobilurines (Flint or Sovran) as soon as possible after the rain stops. Any of these products give excellent post-infection activity, up to 96 hrs. If it clears or at least stops raining tomorrow, and the wind stays down, then there should be time to make those applications where they are needed.

We'll try to get a read on whether the spores were all used up this infection period for the next HF.

High Density Study Group Meeting

June 1, 2001 at 10:00 AM

UMass Horticultural Research Center
392 Sabin Street, Belchertown, Massachusetts
(please call Wes Autio at 413-545-2963 if you need directions)

All are welcome.

We will tour dwarf apple and peach rootstock trials.