Fall Prune Short Course Set

Watch for a brochure and registration materials for our upcoming Prune Short Course this fall (10/27-30/97). These will be sent to you in June and registration received on a first come first served basis.

Prune Reference Date -
There Are Some Heavy Crops Out There

Reference Date Imminent



"Tip hardening", that point where the seed begins to harden at the blossom end of the fruit occurred during the week of 4/21. "Tip hardening" precedes "prune reference date", that point where 90% of the fruits show endosperm development, by about one week.

Reference Date

A "reference date" is a point of physiological fruit development when harvest size can be reasonably predicted by sampling and weighing fruits during the season. Prune's reference date is early in the growing season when 90% of the fruits show visible endosperm development (see drawing). This occurs about one week following the beginning of pit hardening. At this point, ultimate dry fruit size can be reasonably predicted. If unacceptably small sizes at harvest are predicted, thinning practices can be employed.

Prune Reference Date Imminent: Prune reference date, (when 90% of the prune fruits show visible endosperm) occurs approximately one week following "tip hardening" - when the tip of the seed at the blossom end begins to harden. Tip hardening in the Visalia area usually occurs about the last week in April. Begin sampling now to determine tip hardening by cutting through the blossom end of the fruit. When the knife "drags", the seed tip is beginning to harden. Endosperm development, and thus "prune reference date", will follow in about a week.

Sampling directions and tables relating fruit count per pound at "reference date" to ultimate dry count per pound at harvest are attached to this newsletter. Note: We have written procedures for mechanical thinning. If needed, please contact our office at 209/733-6363.

Preharvest Brown Rot Strategy

Due to dry spring weather, blossom brown rot infections are almost nonexistent in local prune orchards. With such low inoculum levels going into harvest, brown rot potential at harvest will likely also be low and brown rot will occur.

That said, a brown rot infection potential still exists if wet weather occurs during ripening. Rovral ®, applied within five weeks of harvest (as fruit touch and begin to ripen), has been shown to provide fruit protection from brown rot. Note: An additional treatment may be necessary if wet weather occurs following 7 to 14 days of a treatment.

Potassium - Monitor for This in July

Editor's Note: The following was information prepared for Northern California prune growers who commonly experience potassium (K) deficiency and are well advised to maintain sufficiency of that element. In the southern San Joaquin Valley, we rarely see K deficiency but may benefit from maintaining higher leaf levels than are currently recommended (i.e. more than 1.3% K in July leaf samples); the best producers of high quality fruit in Northern California maintain leaf levels around 1.8%.

A prune crop will remove about 22 lbs of (K) per ton of dried prunes, plus 17 lbs in tree growth and about 5 lbs in the winter prunings. K is applied in the form of K₂0 (potash). To replace the 44 lbs of K per acre removed, you need to apply 53 lbs of K₂0 per acre.

With a 3 dry ton prune crop, 88 lbs of K would be removed, calling for 105 lbs K₂0 to be replaced. This could be applied as potassium sulfate (54% K₂0) or as potassium chloride (63% K₂0) (not usually recommended). The use of potassium sulfate would call for about 200 lbs per acre.

Unfortunately, much more than this amount must be applied because much of the applied potassium is fixed with the soil particles and not available to the tree.

Research trials suggest that it takes 2-2 ½ times this amount to maintain adequate potassium status.

Following is the best advice for a potassium maintenance program that appears to work:

1. Apply about 500 lbs potassium sulfate (K₂SO₄) annually either by banding or shanking it down each side of the tree row in one direction. Shanking it in is preferred on disced orchards. Do not broadcast. From research, four years of broadcast applications had only moved K down 6 inches into the soil, while banded K has moved 2 feet.
2. Keep track of where you put the potash and apply it each year in the same location. Four or five feet out from the tree trunk has worked well in trials.
3. Apply in the fall (with your fall irrigation is a good time) as soon as leaf drop begins. Also, October-November would be a good time. Growing season applications, as well as late winter applications of potassium chloride (KCl), have given chloride toxicity.
4. Monitor your orchard visually and through July leaf analysis for potassium status. If, after several years, potassium levels in the leaf are well above 1.3% K, you can probably skip a year of application.

Very little is said about the use of potassium chloride, also called muriate of potash (KCl). Growers have killed trees with this product. Growers have also saved thousands of dollars by using it safely. If you are uncertain whether you can safely use KCl, use potassium sulfate (K₂SO₄).

If you can maintain good potassium levels by annual ground applications, why not do it with foliar potassium nitrate (KNO₃) sprays?

Foliar potassium nitrate sprays do have a role in prune potassium nutrition. That role is to add that little bit of K that the trees can't quite get from the soil, particularly in heavy cropped years.

Good potassium nutrition doesn't cost money; it makes money. Prune trees showing no potassium dieback produced a crop valued at $700/acre more than prune trees showing only slight dieback.

Prune Leaf Rust

Prune leaf rust fungus, Tranzchelia discolor, can cause substantial defoliation in local prune orchards (fruit infection is of no consequence). However, preharvest defoliation from rust epidemics, which are felt to be the most damaging, only occasionally occur (as in '95, with late spring rains).

Postharvest defoliation, which is generally the rule in the southern San Joaquin Valley, has not been shown to adversely affect subsequent production or quality (three years of research on the same trees in a block having a rust history in Porterville).

Controlling Rust: Prune leaf rust is controlled with prophylactic applications of sulfur dust, wettable sulfur, or other fungicide (Bravo or Rovral and oil have provided some control in recent experiments) in late spring and early summer before symptoms develop. In the case of sulfur, approximately two months of protection can be expected, indicating that a mid-June application would be best. If wet weather occurs in late May and/or early June, an earlier treatment is suggested.

Leaf Tissue Analyses

July is the best time to assess nutrient status of prune trees using leaf tissue samples. Leaf samples for tissue analyses should be obtained from nonfruiting spur leaves rather than from shoot leaves. Select one or two leaves per nonfruiting spur at random from around each sample tree, in the upper 1/3 of the canopy. A single sample should be composed of at least 50 leaves (I suggest 5 leaves from 10 trees) from the orchard or areas that reflect a possible problem.

July is the month to take leaf samples to most accurately determine an orchard's nutritional status. I suggest this be done annually and fertilizer programs adjusted accordingly. The elements of most concern are nitrogen (N), potassium (K), and zinc (Zn). In special situations, sodium (Na), chloride (Cl), sulfur (S), and copper (Cu) can be problems as well. Note: Iron (Fe) levels in leaf tissue are not useful in diagnosing deficiency of this element. Use visual smptoms and soil analyses for lime content (i.e. lime induced chlorosis). Critical leaf levels are shown in the table on the next column.

NOTE: Be sure to inform your lab if zinc was applied; special washes must be used to remove the surface zinc so leaf content of zinc can be more accurately measured. At best, accurate analysis of tissue topically sprayed with zinc is poor due to surface contamination.

How About Your Water Too?

As long as you are having leaf tissue analyzed, have your well water analyzed too. Of particular interest are the Ecw (electrical conductivity - a measure of salt content), HCO₃- (bicarbonate level), NO₃ (nitrate nitrogen - parts per million, NO₃ x 2.78 = lbs of actual N applied with each acre foot of water), Cl- (chloride content), B (boron content) and SAR (Sodium Assumption Ratio - a measure of the sodium hazard). Note, wells can provide a significant source of N to prune orchards.

Critical Nutrient Levels for Prune Leaf Tissue Leaves are for July Samples

Predicting Harvest Size Of Dry French Prunes - Tulare District

To predict average harvest size of dry prunes, perform these steps:

1. Determine the beginning of visible endosperm development by cutting fruits in half lengthwise with a sharp blade and observing the interior of the seed at the blossom end. Endosperm detection usually starts within about a week after pits have hardened at the tip end.
2. "Reference date" for sampling is when 80-90% of the seeds show the presence of endosperm. On this date, determine the average count per pound by obtaining the weight of 400 fruits (take 20 fruits at random from each of 20 trees in an orchard). In large blocks, do this in several locations and average the results. Sample each tree by taking entire clusters of sound fruit from several locations around the tree.
3. Look in the following table to determine the predicted average harvest size of dry prunes for the orchard sampled. If small harvest size is predicted, one should consider mechanical shaker thinning so as to get the desired harvest size.