Buzz Building About Citrus Seed-Source Trees Under Cover

Rasnake Citrus Nursery seed-source structures in Winter Haven, FL. Photo by Peter Chaires

Rasnake Citrus Nursery seed-source structures in Winter Haven, FL.
Photo by Peter Chaires

The movement of Florida’s citrus nursery industry under screen was nothing short of miraculous. This entire process was not without its challenges and frustrations, but nurseries again demonstrated their resilience, ingenuity, and creativity. To put it simply: They got it done. There are similarities between each nursery, but also many operational differences. Each nursery has its own approach to soil medium, structural design, grafting techniques, potting systems, operational flow, safety and security, and the management of seed-source trees.
Rootstock seed production is now the only element of citrus nursery operations that remains outdoors. Many of these trees are in poor health with ever declining production of fruit and seed. Furthermore, these trees are exposed to canker, which elevates the risk associated with seed extraction. Though many of our future trees will be produced from rooted cuttings and/or tissue culture, there will seemingly always be a demand for rootstock liners produced from seed. The establishment of seed-source trees within screened structures seems the obvious solution, but this is not without its challenges. In fact, several nurseries have attempted seed production under screen and failed.

Motivation To Move Indoors

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Several nurseries and researchers were contacted in preparation for this article. It seems that canker is actually as much or more of a concern as HLB. Nurseries extracting their own seed have strict sanitation procedures to disinfect the fruit prior to extraction, and then sanitize the seed after extraction. However, if trees could be kept free of citrus canker within a screened structure, the risk of canker being introduced into the nursery could be greatly minimized or eliminated. Declining production of fruit and seed as a result of HLB adds to the motivation to establish healthy trees in an Asian citrus psyllid-free environment.

Pollination Concentration

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The physical structure of the citrus flower necessitates the presence of a pollinator for seed production. The physical activity of the pollinator is critical to fruit set. Rootstock trees appear to have no problem blooming under screen, but without a bee to move the sticky pollen from the anther to the stigma, fruit set and seed production will be minimal.

What has failed? Several nurseries have established rootstock trees under screen and introduced honey bees into the structure at bloom. Seemingly, this would produce the desired result. Not so. In most cases, despite best practices to maintain the bee population, the bees died-out without having accomplished their assignment. Why? We don’t know for sure, but it seems that honey bees are a poor choice as a screenhouse pollinator. They have a natural instinct to seek an escape route when sensing confinement. They will fly to the roof and walls and exhaust all of their energy scouting the structure for an exit. In fact, they will forgo even the most attractive flowers in order to find an escape route. Many die never having visited a flower. There was one observation of bees leaving through a small cut in the screen, flying to flowers outside the structure, only to return later to visit the flowers in the structure.

The only incident we found where honey bees were effective was when the sides of the screenhouse were removed during bloom to allow normal native bee activity, and replaced post-bloom. North Carolina jasmine and other bee-attracting plants were positioned outside the screenhouse to lure in the workers. The trees were then sprayed down to kill any Asian citrus psyllids that may have made their way into the structure. In this case, the trees set normal fruit loads and produced normal seed volumes. This method does expose the trees to HLB infection, but the risk of canker isn’t as much a concern during the spring.

Bee-Plus

One possible solution may be the use of a different bee. Dr. José Chaparro, UF/IFAS, reports increased success using bumble bees in structures than honey bees. Apparently, bumble bees lack the escape-instinct and visit the available flowers. Of course, special care must be taken to maintain the hive throughout the bloom period. Bumble bee hives are available commercially. However, there is a downside to the bumble. If left without a food source, they have been known to eat the foliage. Bumble bees are used extensively as pollinators for indoor blueberry crops in other areas of the world.
Dr. Gene Albrigo, UF/IFAS, suggests decreasing the temperature in the structure through the use of shade cloth in winter, but drought stress for four to six weeks should bolster the cool temperature induction and prevent bud break until drought is released. Hand pollination is a more expensive alternative.

Other Interesting Field Notes

  • Chester Rasnake of Rasnake Citrus Nursery in Winter Haven has observed that some rootstock varieties can’t handle the increased heat under screen [pictured top]. They don’t reach sufficient dormancy to bloom. US 812 seemed to struggle in a screened conditions, while Kuharske and X-639 didn’t.
  • More pruning is needed on the trees. They will reach for the roof. Plan pruning so that it does not interfere with the bloom.
  • Two crops can be achieved under screen. Nurseries report one in February/March and another in June/July.

This may be an area of interest for future research. Limited information is available from practitioners experimenting with various methods and practices. However, as Dr. Bill Castle frequently asks: “Has it been documented and replicated multiple times? If not, there is more work to be done.”

Special thanks to Dr. José Chaparro, Dr. Bill Castle, Dr. Gene Albrigo, Chester Rasnake, Paul Harris, and others who contributed to this article.

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