Soil organic matter helps fight HLB on citrus

By Sean Stokes, Thomas Borch and Pankaj Trivedi

The benefits of soil organic matter (SOM) in agriculture have been well documented and include both improved water and nutrient retention. SOM is also essential for a properly functioning rhizosphere microbiome. However, the natural level of SOM in the sandy, acidic soil of Florida’s citrus grove regions is often around 1 percent, a stark contrast to the highly productive soils of the Midwest that typically contain well over 5 percent SOM.

Recent analysis of soil quality by a team of researchers at Colorado State University (CSU) has demonstrated that low levels of SOM in Florida could leave citrus trees more susceptible to the progression of huanglongbing (HLB) symptoms.

After years of unrestricted spreading, it is now assumed that almost all trees in Florida have some level of HLB infection. However, it is unknown why the progression from asymptomatic to symptomatic varies widely. The infection starts in the leaves, but the phloem-limited pathogen, Candidatus Liberibacter asiaticus, is quickly transported to the roots where over the course of months to years it causes fibrous root loss and a shift in the diversity of the rhizosphere microbiome. This means less microbes that cycle nutrients, suppress disease, maintain plant hormone balance and control root development are found in the rhizosphere of HLB-infected trees.

Beneficial rhizosphere bacteria thrive on the various organic compounds (sugars, organic acids and amino acids) exuded from healthy trees through the roots. Twenty to 40 percent of a plant’s photosynthetic products are typically excreted as root exudates. In a tree infected with HLB, the blockage of phloem would hinder the transport of these products from the leaves and lead to significant changes both in the quality and quantity (less in HLB-infected trees) of root exudates. These fluctuations can eventually shift the microbial community in favor of bacteria that more efficiently degrade the larger organic compounds naturally present in the soil. But in Florida, the quantity of this “backup” food substrate is limited and could be quickly diminished to detrimental levels.

At a southern Florida citrus grove more than 10,000 acres in size, the research from CSU focused on one big question: Why do large areas of trees within some blocks (approximately 15,000 trees/block) exhibit a slowed growth rate compared to other trees within the same block?

These tree growth gradients were not caused by management practices, as all trees within the block were treated identically with everything from pesticides to fertilizer to growth activators and were originally planted in the same growing season from identical rootstock. One possibility is that gradients in soil properties were formed during creation of the grove because multiple soil types from different locations were used during the construction.

To determine why these gradients existed, an all-encompassing evaluation was conducted that analyzed every major component of soil quality, including pH, N-P-K, micronutrients, enzyme activity and SOM. The results demonstrated a clear trend that showed the average level of SOM in the “good” soil areas was almost twice as high as the level of SOM in the “bad” areas (0.9 vs. 0.5 percent). An analysis of six common soil enzymes involved in C, N and P mobilization showed that the enzyme activity in the “good” areas was consistently higher and often more than twice the level of the “bad” areas.

We hypothesize that the success growers have had in reviving HLB-devastated groves by adding organic compounds to the soil (through cover crops, compost or biochar) is because they are supplementing the rhizosphere with carbon substrates that the phloem-limiting HLB bacteria have prevented the tree from providing. While it is well known that SOM is an integral part of crop growth, our results emphasize the added importance that SOM management has in a citrus grove affected by HLB, especially in soils with naturally low levels of SOM.

It needs to be stressed that these results are preliminary, so further studies representing more groves in varying citrus regions are needed. A thorough cost-benefit analysis of adding compost, cover crops or biochar should be considered before large-scale commitments are made. However, based on the existing science and the current state of citrus groves in Florida, these results demonstrate the immediate need for field studies by both growers and scientists that views the progression of HLB through an all-inclusive lens that emphasizes SOM and its impact on soil chemistry, plant pathology, microbiology and economics.

Sean Stokes is a graduate research assistant, Thomas Borch is a professor, and Pankaj Trivedi is an assistant professor, all at Colorado State University.