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?

2018-08-09 10.39.14-1
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.

Advertisements

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.

2018-05-22 03.48.30
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.

Photosynthesis and kaolin particle films – Welcome Juanpablo

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.

color buckets

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.

2018-05-22 03.02.04-3
JP Salvatierra applies white kaolin film in one of his experiments.

Effects of kaolin on Asian citrus psyllid

Kaolin films are showing promising results in management of Asian citrus psyllid.  I recently presented preliminary results from our trial of Surround kaolin clay product and a Surround that we have modified with a red dye in presentations to the Polk County OJ Break and to the Citrus Research and Development Foundation research lunch.  To see the complete presentation click here.   

kaolin in field 6 mo
Six-month old trees with white kaolin.

The results are promising: Over the course of the first year after planting we saw an 78% reduction in mean psyllid numbers per tree in the white kaolin treatment.  Thus far, this has also translated in lower infection rates, with a mean of 10% infection in the white kaolin versus 25% in the foliar insecticide treatment.  These results are early, so we should be cautious about jumping to conclusions.  However, other studies have produced similar results, and this means that growers should consider kaolin as a viable practice to incorporate into their management programs.

psyllid plot 1 year
Mean Asian citrus psyllid counts based on weekly counts over the first year after planting on trees treated with Red-dyed or non-dyed (White) kaolin or with foliar insecticide. Click here for full presentation.