Here is an updated guide for herbicides that are registered for citrus, avocado and a few other tree crops.

GAINESVILLE, Fla. — Researchers from the Institute of Food and Agricultural Sciences at the University of Florida are closer to finding a possible cure for citrus canker after identifying a gene that makes citrus trees susceptible to the bacterial pathogen.

Citrus canker, which causes pustules on fruit, leaves and twigs, is a highly contagious plant disease and spreads rapidly over short distances. Wind-driven rain, overhead irrigation, flooding and human movement can spread citrus canker. Human transport of infected plants or fruit spreads the canker pathogen over longer distances.

In Florida, the last extensive canker outbreak occurred beginning in 1995, which led to an ultimately unsuccessful eradication program that ended in 2006. That effort cost an estimated $1 billion and stimulated renewed efforts for more effective and economical controls. Farmers destroyed more than 16.5 million citrus trees between 1995 and 2012.

Yang Hu, a former doctoral student working with Jeff Jones, a professor in plant pathology, found the critical trait in the bacterium that is necessary for disease development.  Hongge Jia, a researcher at UF's Citrus Research and Education Center in Lake Alfred, and Nian Wang, an associate professor in microbiology and cell science also based at the Citrus REC, along with six researchers from three universities worked on the project, as well.

Citrus canker continues to be a problem and exists in most citrus-growing areas in Florida. While scientists like Hu are devoted to eradicating the disease, many other researchers are now also battling citrus greening, which threatens to wipe out the $9 billion industry.

Citrus canker is caused by the bacteria Xanthomonas citri.  While studying the bacterial pathogen's role in infected citrus, researchers were able to identify a gene in citrus critical to the development of citrus canker, known as the susceptibility, or “S” gene.

By finding the susceptibility gene, researchers say they are closer to a cure for the disease.

“The S gene represents an excellent candidate for control measures for the citrus bacterial canker,” Hu said.

Hu and Jones said they hope to secure funding to support further research, and have already identified several genes they believe could be engineered to obtain broad-spectrum plant resistance to most kinds of citrus canker.

“Once you know what the susceptibility gene is, it's possible to design multiple strategies for disease control,” Jones said.

The research paper was published online this month by Proceedings of the National Academy of Sciences:  http://www.pnas.org/content/early/2014/01/08/1313271111.abstract.

Several calls have come in from growers lately about yellow avocado and citrus trees. the yellowing is most common on the late summer flush leaves or can affect the whole canopy on young trees. In severe cases leaves fall. This happens going into winter after a warm fall when growing conditions are good. During the winter, the root systems become depleted of stored starch and die.

During winter, trees go into what is called a “quiescent” state, a version of dormancy found in subtropical tree crops. This is a resting mode that protects them to a certain degree from frost damage. There is not much that can be done in a field setting until temperatures warm up and the trees begin growing again in late winter/early spring. As the temperatures increase, the trees gradually recover and the foliage re-greens.

Winter Yellows can be exacerbated in years when we do not have leaching rains to remove salts from the root zone. And it can also be more severe when we have those years when winter rains just never seem to stop and rootzones become waterlogged. We may never see that time again.

Photo by Greg Moulds

I am amazed how such a simple procedure can go so wrong. For avocado and citrus growers, it’s time to think about planting in the spring. And every spring and summer I get called out to diagnose trees that are failing. It often turns out that the trees have been planted too deeply. There are various ways of killing a tree, such as digging a hole too deeply and then backfilling. When the ground settles, it settles around the root collar and the tree suffocates. Or installing a dry root ball and then not irrigating soon enough. Or adding fertilizer to the planting hole which burns the roots. One of the major problems of lack education and supervision of the planting crew. Don’t assume that everyone knows how to plant a tree.

Deep planting can result in death of woody plants either because they rot in moisture saturated soils or they dry out. In either case, the symptoms are similar: wilting, sunscald or burnt leaves, lack of growth, leaf drop and eventual death of leaves, shoots and branches. Root balls planted below grade cause several problems at establishment. Since native soil surrounds the ball, there is an immediate problem with and interface between the two soil textures.   Most container media are lighter than bulk soil, which is done to make sure there is adequate drainage in the nurse. When these soil-free media are planted in soil which is of a heavier texture, the interface does not allow the water to enter the root ball.

When trees are planted too deeply, they are much more subject to fungal cankers and other pathogens that can girdle the stem, killing it and all above ground parts. Planting slightly higher than grade will prevent this. Just look for the color, textural change between the roots and stem and dig the hole no deeper than the root ball to prevent settling. Newly planted trees can’t draw water from bulk soil, not until the roots move out into the soil will they be able to absorb water. Also planting cannot be rushed, because that’s when errors in planting occur.