Agronomic Technical Bulletin

Wesley Haun, Research Agronomist

CITRUS GREENINGHuanglingbing (HLB, citrus greening) is a devastating disease of citrus crops, affecting all species and varieties. This disease is caused by three strains of a bacteria 1) Candidatus Liberibacter asiaticus which is in the United States and Asia 2) Candidatus Liberibacter africanus found in Africa, and 3) Candidatus Liberibacter americanus present in Brazil, and southern Texas. This disease is vectored by the Asian citrus psyllid (Diaphorina citri Kuwayama). In Africa a different psyllid (Trioza erytreae) transmits the HLB disease. The africanus strain of bacterium prefers cooler temperatures and higher elevations. The asiaticus strain strives in cool or warm conditions and the americanus strain is heat sensitive (Graca, 2013). Citrus greening is present in many citrus production areas around the world. Major citrus production countries with confirmed citrus greening include China, India, Pakistan, Brazil, Mexico, and United States. This disease has resulted in dramatic production losses. Ripe fruit turns green with bitter juice and/or fruit becomes deformed or fruit drops prematurely. Other symptoms include mottling and yellowing of leaves, stunted growth, poor flowering, and dieback.

Asian Citrus Pysllid, USDA-ARS Citrus leaf mottling, USDA-ARS

Off color fruit, USDA-ARS

Agronomic Technical Bulletin

MANAGEMENT OPTIONS

Infected citrus trees generally do not display disease symptoms until one – two years after infection. A long latency period compounds the difficulty in managing this disease. Producers have employed several strategies to minimize the impact of citrus greening on current citrus production. Researchers at University of Florida, Texas A & M University, University of California-Davis, and USDA-Agricultural Research Service have taken active roles in recommending production practices to curtail the spread of this disease. All agree that eradication probably will not occur, but multiple management factors can extend production a few more years to provide a longer window of opportunity for development of longer term control measures such as resistant varieties. Some management strategies utilized include:

1) IPM2) control of Asian citrus pysllid3) scouting for Asian citrus pysllid and infected trees4) biological control methods 5) removing and destroying infected trees6) planting disease free new trees7) establish balanced nutrient management program

Grove location or topography and soil types do not appear to influence instance of citrus greening.

DISEASE TRANSMISSION

HLB bacterium causes congestion of phloem tissue that prevents/reduces transfer of water, nutrients, and carbohydrates from leaves to other parts of plant. Whenever a plant disease is transmitted via an insect vector, there are complex biological interactions between the insect, the plant, and the pathogen (Mann et al., 2012). A research trial was established in a laboratory controlled environment to “demonstrate specific mechanisms through which a bacterial plant pathogen induces plant responses that modify behavior of its insect vector. Candidatus Liberibacter asiaticus bacterium induced release of a specific volatile chemical, methyl salicylate which increased attractiveness of infected plants to its insect vector Asian citrus pysllid and caused vectors to initially prefer infected plants. However, the insect vectors subsequently dispersed to non-infected plants as their preferred location of prolonged settling because of likely sub-optimal nutritional content of infected plants. The duration of initial feeding on infected plants was sufficiently long for the vectors to acquire the pathogen before they dispersed to non-infected plants, suggesting that the bacterial pathogen manipulates the behavior of its insect vector to promote its own proliferation” (Mann, 2012). The Asian citrus pysllid develops through 5 nymphal stages prior to becoming an adult. Pysllid growth cycle is fast as the time frame from egg to an adult is approximately two weeks when temperature is 24-27°C. Adult pysllids aquire the bacterium while feeding on an infected tree and the bacterium persists and reproduces in the pysllid for remainder of its life.

Healthy tree on left and an infected tree on right with pre-mature fruit drop. Photo: U.S. Sugar

* Yield data – Davis Citrus Management.

Graham, J. 2014. Managing Fibrous Root Loss due to HLB. University of Florida & Citrus Research and Development Foundation.

NUTRIENT MANAGEMENT

The nutrient status of infested trees and non-infested trees via plant tissue analyses revealed that the infested trees were significantly deficient in nitrogen, phosphorus, magnesium, zinc, and iron (Mann, 2012). Root health declined as the bacterium translocated to the fibrous roots following infestation of leaves. Functionality of the root system declined as much as 50-80%. Root function appeared to decrease prior to symptoms appearance in the foliage (Graham, 2014). Trees in a weakened condition required greater attention to a balanced nutrient supply. Various plant nutrient programs have been employed to

enhance nutrient supply to citrus trees including fertigation, foliar feeding, and soil applied nutrients as broadcast, or side-dress. Soil pH determines the extent of plant nutrient availability and should be the first adjustment in a plant nutrient program. Many citrus groves have soil pH greater than 6.5 and irrigated with well water high in bicarbonates (> 100 mg/L). Elevated levels of bicarbonate reduces root uptake of Ca, Mg, K, and Fe. These conditions are conducive to greater instance of HLB. Table 1.0 shows the decline in root mass as soil pH increased which resulted in greater levels of bicarbonates.

Grove status No. of blocks surveyed

Root mass density (mg/cm3)

Change in block yield from 2009-12

Low pH stress, Ridge 14 0.6 Increased 6%

High pH stress, Ridge 10 0.4 Decreased 3%

High pH stress, Flatwoods 13 0.2 Decreased 20%

Table 1.0

Relationship between bicarbonate status, root mass density and change in yield from 2009-12*

Maintaining soil pH between 6.3 – 6.8 enhances opportunity for greatest plant uptake of all required nutrients. Figure 1.0 illustrates this concept as the wide bars represent greatest availability of plant nutrients. The greatest availability of all nutrients occurs between soil pH 6.3 – 6.8. As soil pH increases above 7.0 there is significant decline in availability of all micronutrients except Mo.

Figure 1.0

Relationship Between Soil pH and Relative Nutrient Availability

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N

P

K

S

Ca

Mg

Fe

Mn

Cu & Zn

B

Mo

SOIL pH

4 5 6 7 8 9 10

Adapted from Truog, 1946. SSAP 11:305- 308.

Healthy tree on left and an infected tree on right with pre-mature fruit drop. Photo: U.S. Sugar

CONTACT INFORMATION

Sulphur pH Root density (mg/cm3)

NO 6.4 1.1

YES 5.9* 1.4*

References:

Brlansky, R.H., M.M. Dewdney, and M.E. Rogers. 2014 Florida Citrus Pest Management Guide: Huanglongbing (Citrus Greening). Univ. of Florida Extension.

Graca, J.D., 2013. New strain of HLB disease found in South Texas citrus grove. Southwest Farm Press.

Graham, J. 2014. Managing Fibrous Root Loss due to HLB. University of Florida & Citrus Research and Development Foundation.

Mann, R.S. et al., 2012. Induced Release of a Plant-Defense Volatile ‘Deceptively’ Attracts Insect Vectors to Plants Infected with a Bacterial Pathogen. PLoS Pathogens. 8:1-13

Morgan, K.T. 2014. Understanding the Potential Problems with High Bicarbonates in Irrigation Water. Univ. of Florida

Schumann, A.W., J.P. Syvertsen, and K.T. Morgan. 2009. Implementing Advanced Citrus Productions Systems in Florida – Early Results. Proc. Fla. State Hort. Soc. 122:2009.

Spann, T.M. et al., 2010. IFAS Guidance for Huanglongbing (greening) Management. Citrus Industry

* Significant difference P < 0.05J. Graham, 2014, Citrus Institute & Univ. of Florida

Table 2.0 336 kg/treated hectare of TIGER 90CR Sulphur lowered soil pH in 9 months

Reducing soil pH below 6.4 tends to minimize the impact of the HLB bacterium on the uptake of water and plant nutrients even though the root mass has been adversely affected by the disease. Acidification of the soil in the root zone improved uptake of secondary and micronutrients and reduced the bicarbonates levels which adversely influenced nutrient uptake. Soil acidification with sulphur increased root density, reduced fruit drop, increased nutrient uptake and improved general appearance of citrus trees (Graham, 2014).

Research with citrus has targeted soil pH 6.4 as the upper limit to enhance plant nutrient uptake by citrus trees stressed with HLB bacterium. Sulphur is the product of choice to lower soil pH. Two methods have been employed to utilize sulphur as a management tool to enhance nutrient uptake by citrus trees.

1) Water conditioning where irrigation water has been treated with sulphuric acid or N-pHuric acidto reduce the bicarbonate content.

2) Soil amending where Sulphur Bentonite (TIGER 90CR) has been soil applied in sufficient quantityto reduce pH (Graham, 2014).

Water conditioning lowers pH of the irrigation water and reduces amounts of additional bicarbonates introduced to the soil. Soil amending is a longer term benefit by addition of sulphur bentonite such as TIGER 90CR. It functions well where soil pH is high and excessive bicarbonates are present. Soil pH can be significantly reduced and improve root density with application of TIGER 90CR sulphur (Table 2.0).

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