By Dr. Thomas T. Yamashita
The merits of enhanced defoliation with zinc sulfate are proven. In our western soils, zinc is one of the most frequently required micronutrients. Not only is the fall application of zinc sulfate a means of supplementing with zinc, but also an enhancement of dormancy and hardening off of tissues for the winter cold period.
However, a problem arises when we institute this practice on a calendar schedule. For example, a few years ago I observed many orchards defoliating their trees by late August or early September. That year, the fall weather was mild, and would have allowed for additional photosynthesis and food-making by the leaves through early November. Early defoliation deprived trees of 2 to 2 ½ months of additional food production.
Premature defoliation makes trees more susceptible to disease and pest infestations.
Premature defoliation with zinc sulfate sprays is akin to a midseason outbreak of two-spotted spider mite infestations. Many years ago, I ran studies on young cotton plants to illustrate the importance of maintaining active leaves and building food reserves in roots and support tissues. I used the verticillium wilt pathogen (Verticillium dahliae) as a model. While the literature had indicated that vert infections were largely dependent on population density in the soil, we came to find that the health of the host plant was a larger factor.
We planted several cotton plants in soils inoculated with 200 microsclerotia per gram of soil (microsclerotia are resistant stages of verticillium). The plants were placed under varying regimes of light: full light, 75 % light, 40 % light and 25 % light. Another set of plants were placed under varying regimes of leaf pulling: no leaves pulled, 25 % of leaves pulled, 50 % leaves pulled, and 75 % leaves pulled. All plants were watered and fertilized identically. As you might have guessed, plants exposed to 50% or 25% light came down with verticillium. Likewise, plants with 50% or 75% of leaves pulled became infected with verticillium. The plants in full light, as well as those with no leaves pulled, were resistant to infection. This study illustrated the importance of plant health, both in regard to yield and natural resistance to diseases and pests.
Considering the above, one can only imagine the stress placed on a tree by premature defoliation. What is needed to ensure that zinc sulfate spraying is beneficial is the implementation of a monitoring scheme to determine if trees are ready for defoliation. The system I use is to fertilize about 60% to 75% of nitrogen following harvest. Irrigation is maintained to keep trees working. When temperatures begin to drop into the 50s or below at night, I periodically inspect trees. This entails merely running the hand down a hanger or limb. If more than 50 % of the leaves can be easily pulled off, trees are ready for zinc defoliation.
Nutritional treatments can help protect trees from some consequences of defoliation.
As an additional protection, I often add some fungicide as an early protection against shot-hole and leaf curl. However, the zinc defoliation we use relies upon chelated zinc and the introduction of calcium and boron (where boron is deficient). We have observed that excessive use of phytotoxic zinc sulfate sprays can create weak spots on young, current season shoots and buds. These weak spots can oftentimes be the initial point of freeze damage or colonization by pathogens (e.g. Ps. syringae, scab, anthracnose, etc.). Heavy use of any metallic element can disrupt tissue and membrane integrity.
A brief aside for the technically minded: One consequence of heavy metal exposure is the displacement of calcium from its normal position in membranes and cell walls. The atomic volume of calcium is approximately 40 cc per mole, whereas most metal elements runs are around 7 to 10 cc per mole. Thus, replacement of a site normally occupied by calcium with zinc or other metals can skew the 3-dimensional orientation of the tissue, disrupting its normal, efficient operation.
Boron, like calcium (but to a lesser degree), also plays a role in membrane and cell wall integrity. Thus, when the time comes for defoliation, many of our growers are encouraged to use programs such as the following example. Please note that volumes listed are per 100 gallons.
- UAN-32 – 3-4 gallons
- Complexed Calcium – 1 gallon
- Complexed Zinc-Manganese – 1 gallon
- 10% Boron – 1 quart
- Fungicide – recommended rate
- Silicone Surfactant – recommended rate
Note that the additional nitrogen is instrumental in facilitating the rapid decay of leaves, which may host various pathogens. It is also needed for additional storage of nitrogen in the buds. The use of silicone based surfactants adds a great deal to the efficiency of the spray. The addition of calcium and boron are insurance for maintaining high tissue integrity, which is especially important going into the winter dormancy.
As an added note regarding the program above, it is always best in a disease control program to reduce the inoculum levels of the pathogen. This is part of the reason for including a broad-spectrum fungicide early in the season, and especially along with a superior surfactant. Many have been led to believe that sticking quality is important for sprays applied prior to or during dormancy. The problem is that most sticking agents, including oils, have inferior spreading capabilities when compared to silicone-based surfactants.
We have found that it is always more effective to secure complete coverage of tissues and sacrifice sticking quality, versus achieving better sticking with only partial coverage. One benefit of improved coverage is that, for example, many of the spores of shothole will lodge within the bud scales, a tight and well-hidden environment that can only be reached with superior spreading agents. If your trees are defoliating despite your fertilization and irrigation after harvest, you should have your soil inspected for nematode and water mold pressures.
Above all, do not promote premature defoliation of your trees and vines. I have, for example, observed many prematurely defoliated orchards experience poor fruit set the following spring. This is especially true of plums, prunes, apples and nut crops. In varying degrees, the same weakening effect is observed on other tree and vine crops. Keep the trees and vines actively harvesting sunlight for as long as the season allows. The concept is not unlike how a marathon runner prepares for the race. Instead of skipping breakfast, lunch, and dinner the day before the race, a runner eats 3 square meals and even loads up on pasta for dinner.
