There has been discussion lately about how effective antibiotics that are sprayed on citrus groves might be agains the bacterium (Las) that causes citrus greening. In order for these to that they need to be moved in the vascular system, particularly the phloem, where Las hangs out. So, are leaf-sprayed antibiotics are moved systemically?
This is the question some colleagues and I asked in a recent paper. We were following on some of Nian Wang’s research that showed how trunk-injected oxy-tetracycline (which we’ll affectionately call “oxytet”) moved around the citrus plant, eventually reaching all parts. We had a few questions:
When oxytetracycline is sprayed on the leaves, does it move to other parts of the plant? This question addressed whether there is systemic movement at all.
How much moves? If there is systemic movement, what proportion of what is sprayed is moved.
Does it matter whether you spray on old or young leaves? Some growers try to spray when there is new flush? New flush has thinner cuticle (the waxy layer on the outside of the leaf). We thought that applying to new flush would allow more oxytet to get into the leaf.
In the same study we looked at some of the effects of heat treatment, but today we’ll just look at oxytet delivery. These trees did not have HLB, this is because we wanted to look at oxytet movement, not efficacy against Las.
What did we find?
Oxytetracycline did move into leaves that did not receive the spray.
How much moved? In one trial we found between 0.36-0.63 of the concentration in the leaves that didn’t receive the application relative to the leaves that were sprayed. But in the next trial we found between 0.27 and 0.34. So, although we did find oxytet moved into leaves that weren’t sprayed, a portion of what was sprayed stayed in the original leaf. The portion that didn’t become systemic probably is useless against Las because it isn’t moving through the vascular system.
It doesn’t matter whether you spray old or young leaves. We found the same concentrations in the leaves regardless of which leaves were sprayed. At first we thought this contradicted some previous work that showed that there was a decrease in delivery of nitrogen as leaves aged. However, taking a closer look at that older work, it turns out that delivery decreases from when leaves emerge until about 6 weeks. However, after this point delivery begins to increase again. This is because, although there is more wax on the outside of the leaf, this wax ages and forms cracks, which probably allow more of whatever is sprayed in.
How did we find this?
Some plants we removed all new flush, some we removed all old flush, and some we left all the leaves. We covered about 1/4 of the canopy of small trees with impermeable plastic. Then we sprayed the rest of the canopy. After the spray had dried, we removed the plastic. About 3 weeks later we sampled both the leaves that were directly sprayed and those that weren’t. Then we tested each for oxytetracycline content.
Is it enough?
We still don’t know whether the concentrations that made it do the unsprayed leaves were enough to reduce the Las levels, because we actually don’t know how much oxytet it takes to bring Las down in the plant. This study didn’t address streptomycin, the other antibiotic that is labeled to use against Las, so we don’t know whether it would move similarly.
Because the life cycles of two important pests, Asian citrus psyllid and citrus leaf miner, are closely linked to the emergence of new flush. Growers could benefit from spraying only when there is new flush. But if the late flush is spread out over a month or even more, then what’s a body to do?
There may be ways we can manipulate the timing of flush directly. But to do that we need to understand how the plant regulates its own growth patterns.
Citrus trees have a different growth cycle than most common fruits. Like many fruits, they flower in the spring and then fruit develops over the next several months. Deciduous plants add vegetative growth (leaves and shoots) during a solid, extended period of the year. Citrus plants grow in several cycles of vegetative flushes over the year. The number and the size of these flushes depends on the environment. In Florida there are 2-3 vegetative flushes per year. Usually this begins with a flush in May, with later flushes in July and September or October. These flushes become less coordinated from plant to plant in a grove as the year waxes on, with May flush being relatively concentrated and the late flush being very sporadic.
Citrus plants’ “distribution of wealth”
The plant has to balance the needs of the leaves for water and nutrients with the needs of fruits and roots for sugars. This gets even more complicated when the plant has to send sugars to a new shoot to new flushes that will need to grow more than 1 cm in a day. So the plant needs to to start flushing at just the right time so that its fruits and roots don’t starve. It also needs to not start a new flush when the old one is not yet mature. What researchers in the 1990s observed is that root growth stops when shoots are growing.
How do they do it?
We humans have the benefit of brains that coordinate all the different ends of our body, so that our heart beast the right amount for our feet to be able to run. Plants don’t have benefit of brains, so they rely on chemical signals. These signals can be molecules that move from roots to leaves or that accumulate in one part of the plant when the other part stops using that compound. Most of these signals are called phytohormones, because their effects are very large compared their very small concentrations. Growing shoots send out signals called auxins that keep other buds from growing so that there aren’t too many flushes. Meanwhile, gibberelic acid and cytokinins coordinate between roots and shoots. Not all of these dynamics have been completely explained in the case of citrus flush phenology, but we are gaining ground!
Why does it matter?
Knowing the “code” the roots, leaves, and shoots use to communicate, allows us to grab the microphone and give a few orders. Thus it may be possible to manage the flush timing, to decrease its sporadic nature and improve the efficacy of our pest management, ultimately getting healthier plants with greater yields.
Because the Asian citrus psyllid stakes its reproduction on new citrus flush, there is a lot of interest in tailoring management to citrus phenology. “Phenology” is an uncommon word, but it boils down to how plant development changes over time. For instance the development of the spring flowering flush is a phenological process and names like “feather flush,” “popcorn,” and “full bloom” describe phenological stages.
Gene Albrigo has been involved in phenological modeling to predict flowering intensity and bloom time since well before the HLB era. He has recently turned to using this model to help improve psyllid management in two ways: reducing psyllid reproduction on new flush through pre-emptive psyllid management, and reducing negative impacts of insecticides on bee pollinations. In other words his goal is to kill adult psyllids before they can lay eggs on tender new flush but not hurt pollinator bees with applications late in the flush, when flowers have emerged. This can be done by using the models he and collaborators developed and have maintained for more than 10 years.
Gene has worked with several regional growers, selecting some blocks to manage psyllids based on phenological predictions, leaving others as controls with calendar or sampling-based sprays.
Gene recently reported results from the first two years of developing this approach. Results are positive, with reductions in adult psyllid numbers and egg-laying using the phenology-based approach, spraying once just prior to budbreak and again about 4 weeks later. This also allowed a bloom period that was free of insecticide applications, leaving the pollinators to range at the appropriate time. These results are promising for psyllid management during the floral flush, and I expect this approach to expand to become a standard practice.
Kaolin particle films are having promising effects in managing Asian citrus psyllid, but they also have effects on photosynthesis. To help dig into this, a new member has joined our lab: Juanpablo Salvatierra Miranda, or “JP.” He’s 4 months into his first round of experiments, and he’s already made some important observations. He’s focusing on the how photosynthesis changes over the course of a day – “diurnal photosynthetic dynamics” – in response to kaolin particle films of different colors.
JP comes most recently from his native Chile, where he was working for a private agricultural research company. He has experience in horticulture of vegetables, wine grape, and citrus. His Master’s thesis will consider how different colored films affect growth and photosynthesis, as well as how these affect the development of huanglongbing symptoms in the field.