Article prepared by the technical team of Avium SpA.
Leaf fall is not just a sign that autumn is coming; it marks the beginning of an essential stage for the tree’s rest, protection against winter, and preparation for the upcoming spring budding.
In addition to marking the start of the winter dormancy phase, leaf fall is very relevant because it signifies the beginning of the accumulation of chilling hours, a necessary requirement for proper budding/flowering that will help the cherry tree reach its maximum productive potential.
From a phenological perspective, the homogeneity of this state in the plant is associated with future homogeneity in budding. Therefore, it is crucial that management practices preceding this action encourage a normal entry into dormancy, as these will benefit future production.
From the point of view of tree phytosanitation, leaf fall creates wounds due to leaf abscission, which are gateways for pathogens. Therefore, these should be protected throughout the process by implementing a preventive foliar application program aimed at reducing the incidence of diseases such as bacterial cancer and various fungi that cause wood diseases, which often operate silently and are the primary agents of sudden plant death in spring.
When to Encourage Leaf Fall
On the other hand, it is also key to reach May 1st with at least 50% leaf drop, a visual indicator that, from a physiological perspective, represents the start of dormancy and coincides with the moment when, from a climatic perspective, chilling hours begin to accumulate.
Does leaf fall happen naturally, or can it be promoted? There is a transition process from fully active leaves to their fall. Within this phase, there are some cultural practices, such as gradually decreasing irrigation frequency until it is stopped, to encourage this leaf drop, as well as discontinuing the stimulation of the tree with any type of fertilizers, either foliar or soil-based. Stopping the stimulation of the tree also encourages entry into dormancy.
Ideally, leaf fall should occur naturally. However, conditions are not always favorable for this. When phenomena like low temperatures, frost, or precipitation events occur, they will trigger natural processes in the plant that cause leaves to detach. If this does not happen, more aggressive measures are needed, which translates into using a mixture of Urea and Zinc sulfate to accelerate leaf drop.
Importance of Chilling Hours and Their Relationship to Leaf Fall
What are chilling hours? Essentially, the concept refers to one of the most widely used models for quantifying cold. The chilling hours model (Weinberger, 1950) is cumulative and corresponds to the sum of hours with temperatures below 7.2 °C. There are also other agroclimatic indicators for quantifying cold, such as chill units (Richardson et al., 1974), and the dynamic model (Fishman et al., 1978), more commonly known as chill portions, among others.
Meeting the chilling hour accumulation requirements based on the standards of each variety/rootstock combination is an important milestone directly related to productive potential. A good accumulation of chilling hours will be reflected in the cherry tree waking up and expressing itself properly in spring.
Chilling hours are counted every year, starting from May 1st and ending on July 31st; this is the optimal period to consider, quantify, and compare chilling hours. At the same time, it is critical to visualize the accumulation dynamics and analyze from the second week of July whether 70% of the chilling hour requirements have been met or will be met, aiming to make informed decisions regarding dormancy breaker applications.
Given that the plant begins to accumulate chilling hours from the moment it effectively enters its physiological dormancy, it is very important to take advantage of periods of greater accumulation with the leaves already fallen. Otherwise, this could mean that when quantifying “effective” chilling hours (starting from 50% leaf fall), the minimum requirements may not be met, especially in areas with earlier production, where such accumulation tends to be more limiting than in others.
Timely leaf fall is ultimately related to a greater potential for accumulating chilling hours, more uniform budding, and other factors that contribute to achieving productive potential.
