se·nes·cence
the condition or process of deterioration with age.
· loss of a cell's power of division and growth.
Senescence (yellowing and death of leaves in fruit trees) in plants is a highly dynamic process that is precisely coordinated by a complex regulatory network in response to endogenous developmental signals and environmental cues. Certain features characterize the onset of senescence. The cells show degenerative changes often associated with the accumulation of breakdown products. Metabolic changes accompany the degeneration. Before senescence occurs, however, there is an opportunity for fall nutrient resorption! Adversely, there are also variables that limit the trees ability to effectively absorb nutrients. For instance, low soil moisture has been shown to reduce the efficiency of N and P resorption from senescing leaves. Additionally, reduced photosynthesis as a result of spider mites can hinder resorption of nutrients, as nutrient storage comes at the cost of carbohydrates.
We’ll focus on the studies regarding fall time nutrient benefits. Research on foliar application of urea dates back to the 1940’s in California. 1990’s work shows that a foliar spray of urea on peach begins to move into the leaf within two hours, with over 80% of the applied N moving into the leaves within 24 hours. Dr. Eric Hanson, MSU nutrition expert states that the majority of the uptake occurs in the first 6 hours after application. The efficiency of N recovery through a foliar urea application is four-fold greater than through a soil application. (If you're trying to cut back on your annual fertilizer expense this is a program that will get more N in the tree for fewer dollars!) This nitrogen is mobilized and moved into other plant parts such as shoots and buds within one week. Trees move nutrients from the leaves back into the buds and wood prior to leaf drop. It remains there until spring when it is available for early use during cell division (more cells = larger fruit)!
Peach fruits have three fairly discrete stages of growth. The first stage (Stage I) lasts from full bloom until about 50 days after bloom. During this time the fruits grow fairly rapidly and growth is primarily due to cell division. Most of the cell division occurs during the first 30 days after bloom, but the length of stage I may be influenced by temperature. There is an increase in both fruit size and fruit dry weight. During stage I, shoot growth begins but there is too little foliage on the tree to support the growth of the fruit and shoots. Therefore much of the carbohydrates for early fruit and shoot growth come from reserves stored in the tree during the previous season. Urea is known to enhance the uptake of other micronutrients when sprayed in combination. No detrimental effects have been found on winter hardiness, quite the opposite, research has shown that healthy buds with good nutrient reserves are the most winter hardy. Now is a great window of opportunity to put a bit of fuel back in the tank for next year.
Dr. Greg Lang of Michigan State University has found that late summer or autumn urea sprays increased the shoot hardiness of the cherries that he tested and produced up to 20% larger spur leaves in the spring. As a whole, throughout the growing season, the spur leaves are the most important leaves for supplying nutrients to developing fruit. Greg speculates that if the spur leaves are larger, photosynthesis is increased and there are more carbohydrates being exported to the developing fruit. In his trials Greg applied two applications of urea as a foliar spray. An application on August 31 and a second application about one week later actually gave the best uptake of N into spur tissues and provided earlier acquisition of cold hardiness in the year that it was treated. However, application can be made up to leaf fall.
Each application should consist of 15 to 20 pounds of Urea product per acre. Higher rates have shown good effect and no phytotoxicity. I would encourage you to use the rate that you are most comfortable with. Dilute sprays of 250 gallons/acre are possible, but some leaf burn at the leaf margin could be possible with these dilute sprays. Concentrated sprays ranging from 25 to 100 gallons/acre show less phytotoxicity. The reduced toxicity which was noted with the concentrated sprays is probably due to less pooling of the material along leaf margins and therefore less burning of the foliage.
Organic growers have available to them a great plant protein based nitrogen product called Explorer. To recap, Explorer 16-0-0 is a 100% water soluble nitrogen containing compounds of protein, peptides, amino acids and has a 40% carbon content. Explorer stays in suspension nicely and is suitable for soil or foliar application. We’ve added a liquid Explorer 10-0-0 to the lineup as well! There are four forms of Nitrogen; nitrate, ammoniacal, urea and protein. Protein nitrogen does not build up or become toxic in soils, nor does it leach and create concerns about ground water contamination. There’s also no risk for leaf tip burn or volatilization. An added benefit is…no more fishy smell! Keep it in mind for your organic Nitrogen option.
There are two optimum timings for boron application. In the fall while the leaf is still active, or in the spring once vegetative growth has started BUT before bloom.
Boron is one of 16 essential elements for plant growth and development. Soluble, or available boron is very easily leached from the soil. Soil pH has a significant impact upon the availability of boron to plants. Adsorption (binding to) of boron to soil particles rises as soil pH increases from 5 to 9. Essentially this means that the higher the soil pH, the more tightly bound the boron is to the soil particles. Boron becomes increasingly unavailable to the plants as the pH increases. Boron is also tightly held by organic matter within the soils and consequently as organic matter levels increase in soil, boron availability decreases.
The role of boron in plant nutrition is still the least understood of all the mineral nutrients. There is a long list of postulated roles (scientific guesses) of boron: (a) sugar transport; (b) cell wall synthesis; (c) lignification; (d) cell wall structure; (e) carbohydrate metabolism; (f) RNA metabolism; (g) respiration.
· Horticultural Science, NC State University
Boron plays a very critical key role in cell wall synthesis. In boron deficient plants the cell walls are dramatically altered compared to cell walls of boron sufficient plants. Disorders such as ‘cracked stem’ ‘stem corkiness’ and ‘hollow stem disorder’ are all caused by low boron levels. Boron deficiency is a very widespread nutritional disorder. There are many factors that can contribute to this problem, among them being: (a) boron is readily leached under high rainfall situations (sprinkler irrigation) (b) boron has limited mobility within many species of plants; (c) boron has decreased availability with increasing soil pH; and (d) availability of boron also decreases significantly under drought conditions.
In fleshy fruits that are boron deficient, the growth rates are lower and fruits are smaller. Fruit quality can be severely affected by malformation. The demands on boron for reproductive growth are much greater than the demands for vegetative growth. This is directly tied to the need for boron in pollen tube growth. In some plant species, poor growth of pollen tubes results in parthenogenesis (production of a fruit from an unfertilized egg). This is particularly true for grapes. Parthenocarpic fruits remain very small and are of very poor quality. Crops differ in their sensitivity to boron deficiency. Boron deficiency of grapes is one of the most severe diseases in vine growing. Fruit formation is impaired and yield depressions as high as 80% may occur compared with plants adequately supplied with B. This is a consequence of the high requirement of boron for pollen tube growth and viability.
The most common timing to apply ground fertilizer has been to make an application in the late fall. Research has shown that uptake and movement into the tree of a dormant application is greatly reduced and subject to winter leaching (recovery of soil-applied N by fruit trees typically ranges from 25-35%, Khemira, 1995). Moreover, a late fall application will not be available till late, (too late) in the spring, past the cell division period when soil temps warm up. A dormant application contributes most toward summer vegetative growth. Consider a different timing. Mid-August to mid-September most terminal growth has ceased, the temps are still warm and the tree is maintaining a high rate of respiration. Fertilizer applied at this time, followed by a light irrigation will quickly be moved up into the tree. It’s then stored, ready for cell division the next spring. The “sweet spot” to aim for is the period when the tree is between its summer work and winter sleep. Be cautious about late maturing varieties, they stay active later in the season.
“Agriculture is the most healthful, most useful and most noble employment of man.”
- George Washington
Although not completely absent, disease issues were far less of an issue this season. An ounce of prevention is worth a pound of cure! It’s hard to wrap our minds around the attitude of being proactive, not reactive. The materials we have available are protectants, not curatives. So what’s your plan to have clean fruit next year? The following is taken from the UC IPM website – “Shot hole is managed primarily with fungicide treatments to protect buds and twigs from infection.” The key word in successful blight control is “prevention.”
The first autumn rains can start the spread of the disease. Infection can take place in the dormant season if proper moisture and temperature conditions occur. Spores, spread primarily by splashing water, can remain viable for several months. Establishing a protective barrier with copper is vital to keep Coryneum from germinating and spreading. Good spray coverage is important! The disease usually starts low inside the tree where moisture persists, so be sure to target this area. The most common application timing is at 50% leaf fall. It’s not necessary to wait for that to happen. How aggressive you need to be with rates and applications depends on if you need to clean up a problem or are just performing routine maintenance.
Sanitation is a key component of any disease control program. In orchards where twig infections are prevalent, the efficacy of the dormant treatment can be improved by pruning out and destroying infected wood. A conscientious annual program of removing infected wood is necessary over multiple years to alleviate the problem. The following quote comes from the 1996 Colorado Tree Fruits crop management guide. “Once established in an orchard, Coryneum blight is difficult to eradicate. Infected buds and twigs may produce spores for 2 to 3 years. ”
How aggressive you need to be depends on the level of infection you had this season. We may be back to our normal two fungicide season, but if it rains……..
“Most things, except agriculture, can wait.”
- Jawaharlal Nehru
Weed control in the tree row is much more than cosmetic. There is a measurable benefit to tree growth from the reduced competition for water and nutrients. Mouse populations are lower when there is little or no weed cover for them in the tree row The most economical time to control weeds, regardless of the material that you choose, is in the fall before leaf fall or with the leaves raked away, up until the ground is frozen. This application will control the fall germinating annuals that otherwise will require treatment in the spring...your VERY, VERY busy spring! It is always easier and cheaper to prevent a problem than to cure it.
The constant factors in safe and effective weed control are calibration, uniform coverage and timely incorporation of residual materials into the soil by irrigation, snow or rain. Some residual herbicides can injure trees if the application rate is higher than the immobilizing ability of the soil and they are carried deep enough into the soil to contact tree roots. In other words, light soils are more risky than heavy soils. I know that a number of you use backpack or 4-wheeler tanks to spray weeds with contact herbicides. Using a hand wand to apply residual materials is risky at best. It’s next to impossible to get a uniform, proper amount applied. Some labels do not allow for application with a hand wand. Check to be sure your nozzle style and configuration delivers a uniform pattern on the soil surface. Account for the overlap that you will have in the center of the tree row. Don’t double the actual rate of applied material in this area by hanging extra nozzles or increasing nozzle size on the end of the boom.
Remove large weeds that prevent uniform spray coverage of the soil surface, the ‘shadow’ from existing weeds will be where your weed control will fail first next year. Do not spray over the top of a heavy leaf drop; many of the small germinating annuals will be protected from the contact materials in the mix. If a windstorm moves the leaves before the next good rain, your residual material may be gone also. The cleaner the soil surface at the time of application, the more effective the material will be. Organic matter on the soil surface will bind up some of the material before it can get into the soil. If you still have irrigation water available, incorporate the material soon after application with an irrigation of ¼” - ½” . This will ensure no weather degradation and allow the residual control material to bind close to the soil surface where it is most effective.
“No race can prosper till it learns there is as much dignity in tilling a field as writing a poem”
-Booker T. Washington
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