Winter 2021
Determining causal agents that threaten orchard health
From disease to pests to water, maintaining orchard health runs the gamut. While some orchards are outfitted with technology that allows them to regulate and monitor certain aspects of the field, this doesn’t necessarily mean problems in the orchard will be accurately diagnosed, if diagnosed at all.
There are a number of factors that can cause issues in an orchard, and these fall under one of two broad categories – biotic (living) and abiotic (nonliving). To further complicate diagnosis in the field, either of these categories can predispose a tree to the other category. For example, extreme heat can stress a tree, thus making it more susceptible to pest infestations. Being able to accurately and successfully diagnose problems in an orchard can be an involved process, but a few necessary steps can help with accuracy.
Spotting a problem
Diagnosing problems in the field isn’t a one-person mission, nor should it be. Getting more people involved – whether they be other growers, pest control advisors, certified crop consultants, Extension farm advisors or diagnostic personnel – helps avoid premature conclusions or a potentially single-minded approach by offering other perspectives, experience and expertise. This is much easier for larger operations with full staff, but equally important for smaller farms to draw on available experts.
In order to diagnose an abnormality, a grower must first recognize that there is, in fact, a problem. Simple observation – whether through the naked eye, specialized equipment detection, or changes in yield records – plays a significant role in the diagnosis process.
When a grower knows how a healthy tree should look and behave, changes that indicate something problematic are caught more easily. This emphasizes the importance of growers knowing their orchards, recognizing what healthy trees of various cultivars should look like, and becoming familiar with the signs and symptoms of disorders that can occur in their trees.
Even if it seems mundane to walk through a field in a world where agtech is a constant assistance, being physically present in the orchard on a regular basis can provide incredible insight to the goings on within an orchard – issues that new technologies can miss entirely.
“Growers may encounter a situation where they have a sensor they’re relying on, and that sensor stops giving real information,” said Richard Heerema, pecan specialist with New Mexico State University Cooperative Extension. “If the sensor is monitoring soil moisture and gives a consistent, flat reading, a grower may not turn on irrigation because it appears that there’s water there. In reality, the moisture is going down. Without physically getting into the field, growers might never know.”
Inductive reasoning
Symptoms are signs that something is amiss with the health of the trees, and they develop because the causal agent produces – or causes the trees to produce – toxins, enzymes, or growth regulator imbalances that hinder cellular function. Since symptoms are often associated with specific cellular functions, they can be helpful in identifying the problem and developing a solution.
One step further is symptom expression, which can be unique to certain causes. Chlorosis, for example, is a yellowing of the leaves, and how chlorosis is exhibited can help point to the cause. Chlorosis can occur generally over the whole leaf, or it can be interveinal (yellowing between the veins) or veinal (yellow veins).
“Looking at chlorosis this way can help determine what a grower is looking at. It can help decide if it’s a micro- or macronutrient deficiency based on the pattern,” Heerema said.
Basal chlorosis – where the older leaves are chlorotic, but the younger leaves remain green – can tell the observer that the tree has remobilized the nutrient from the leaves on the base to those that are newer. This eliminates some usual suspects since not all nutrients are mobile, and helps growers zero in on what the causal agent is. In this case, it could be a nitrogen deficiency.
But some deficiencies express themselves in similar – if not the same – ways. Iron is a classic culprit for interveinal chlorosis, but a manganese deficiency might show similar symptoms.
“You have to use other clues to help you,” said Heerema.
If symptoms are expressed by spreading from an injury site, appearing in specific parts of the plant only, or spreading through the tree canopy in a directional manner, such as top-down or bottom-up, growers can eliminate possible causes based on this type of observational information.
This is also a good time to take a camera into the field to photograph the visible symptoms and the stage or severity at which they appear.
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Diagnoses, however, can’t be based on symptoms alone, as causal agents can affect multiple plant processes simultaneously, and those symptoms can change over time. As a condition worsens, symptoms may intensify or change altogether. Those same symptoms may also appear differently among trees for a variety of reasons, including the tree’s age and growth stage, the environmental conditions during infection, or the virulence of the pathogen infecting it.
Gathering information from the field as a whole can be incredibly useful to the information based on symptoms. Every orchard has a history, and knowing what that is – in terms of crops and prior use, inputs, irrigation, pests, disease and more – can provide a significant amount of critical information in trying to identify a problem.
For example, some rootstocks and cultivars are more susceptible to certain pathogens than others, as are trees undergoing a certain stage in the production cycle, and this can be a reason why certain abnormalities are showing up. All the information a grower can gather about his or her field is helpful information, including cultural practices, history of the field, rootstock and cultivar, weather conditions, production and yield history, and whether or not activities on the adjacent properties are helping or harming the orchard in question.
The idea is to view the orchard as a whole, both above and below the ground, and to consider all the variables. By stepping back and gathering this type of critical information, a grower can see the spatial variability of the abnormality, how much of the orchard is affected, if other trees or plants are affected, how the patterns are appearing in the field and how quickly they’re spreading.
