How do citrus trees ‘decide’ when to flush?

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?

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Asian citrus psyllid nymphs feeding on young flush, not old leaves.

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.

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Coordinating psyllid management with citrus flush

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.

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A May feather flush with psyllid damage.

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.