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4 Citrus diseases and their management


Hello! Welcome this is the fourth talk of the week. In the last talk we have talked about the important diseases of grapes and their management . Today we are going to talk about important diseases of Citrus and their management citrus is also encountered by a series of pathogens some of them are considered to be highly damaging and those includes Citrus canker, Gummosis, Tristeza and Greening. These are some of the most important citrus diseases and they cause severe damage to the Citrus growers. Let us take one by one and see what are the causal agents.

Citrus canker is caused by a bacterium known as Xanthomonas axonopodis pv. citri.  The disease caused symptoms on all aerial parts of the plant right from leaves, to fruits, to twigs, to thorns including the bark. So the disease is very well manifested through development of cankers on the leaf surface or on the fruits. These are hard corky layers and they are easily identified on the infected plants. The pathogen survives on these cankers present in leaves, stem and fruits and if they fall on the ground their survival rate is lower in comparison to their survival on intact plant. So intact plant management is the most important management aspect that helps in managing this particular pathogen.

IPM practices has been recommended for managing this pathogen that includes cultural and chemical methods, pruning of badly infected twigs before the onset of monsoon is one of the most important cultural control measure that is recommended for management of the disease. Since the pathogen is present on the infected leaves and twigs that is why pruning of this infected plant parts is very essential. It is also advisable that wind helps in spreading of the pathogen from one plant to another plant that is why a wind break in the border of the orchard is quite helpful and that reduces the wind speed and thereby it can have a good impact on the plants form there by checking the spread of the pathogen significantly within the orchard. Some of the chemicals are also recommended for management of this particular pathogen that includes Streptomycin sulphate that is at a rate of 500 to 1,000 ppm or Phytomycin at the rate of 2500 ppm or copper oxychloride at the rate of 0.2% at fortnight intervals. Application of these chemicals fortnightly significantly reduced or the spread of the pathogen from plant to plant. Then there is also an important aspect that controlling the leaf miner in young flush particularly is recommended because they can also transmit the disease from the older tissues to the newly growing tissues. So these chemical methods along with the cultural practices are essential for management of this particular disease caused by Xanthomonas axonopodis pv. citri.

The next important pathogen of Citrus is Gummosis. It is caused by a variety of Phytophthora species that includes Phytophthora Parasitica, Phytophthora palmivora and Phytophthora citrohthora. So all these phytophthora species can cause this particular disease and mostly the disease is manifested on the trunks of the citrus trees where there is extensive rotting and damage under trunks and there is a quite significant amount of exudates that come out from these infected tissues. So this particular disease is identified based on this rotting at the foot along with excessive amount of Gummosis that takes place from these infected areas. The pathogen survives as oospores in soil. So this is important that the pathogen should be managed at the soil for having a reduced inoculum for infecting new host.

Several IPM practices has been recommended and some of them are cultural and chemical in nature and that includes since, the pathogen can enter the host if the plants are injured or the crown are injured. So avoiding this injury to the crown root can serve as a good cultural practice for reducing the disease development. That is normally it is observed that during cultural operations there is every chance of getting the crown root or base of the stem get injured and that facilitate the entry of the pathogen to cause the disease. If the lesion has girdled less than half the girth then removed the diseased bark with a knife along with half un-invaded bark. This is how we can check the progress of the disease further by removing the bark partially the infected portion along with the partially healthy portion that ultimately leads to removal of the pathogen from the bark. The chemicals that proved effective during these management practices that include application of Bordeaux that is a mixture of Copper sulfate, lime and water in the ratio of 1:1:10 that means 1kg of copper sulfate in 1 kg of lime in 10 liters of water when they are mixed and applied then there is good control over the pathogen is observed. So this chemical method along with cultural practices can be adopted for successful management of these particular disease.

Next is Citrus Tristeza. It is caused by a virus known Citrus Tristeza virus and the symptoms of the plant source in all parts of the plant including fruits, leaves and the inner stem portion. So there is yellow mosaic like symptoms that are present on the leaf and the inner stem portions they are basically pitted. So this is a typical symptom of the pathogen causing the disease and ultimately these symptoms if they become severe they may finally cause drying up of the entire plant. So one can imagine that how dangerous or how threatening this particular disease may be because it can lead up to 100% loss from that infected tree. The pathogen since it’s a virus it remains in the infected citrus. So management of the infected trees is the most important management practice.

IPM practices include mostly the cultural practices and because being virus no chemicals are found effective to manage these particular viral disease and cultural practices include avoiding susceptible root stocks. This is one of the most important cultural aspect that needs to be adopted for particularly sweet orange and mandarin cultivation. So susceptible rootstock may lead to development of the disease caused by Citrus Tristeza virus. Then seedlings should be pre immunized with mild strain of tristeza. This is a method that is very skillful and it can be done only under commercial cultivation where skilled persons are available. So there are certain mild strains of tristeza identified. If this mild strains of tristeza virus is inoculated into the plant system then following challenge with the virulent strained of citrus tristeza may be reduced because of the presence of the mild strain. the my strain already present in the plant lead to a mechanism that is highly suppressive in nature and when the virulent strain infect the plants then this resistance level that is generated through the mild strain are effective in lowering down the disease development.

Next important disease of citrus is Citrus Greening. It is caused by a bacterium Liberibacter asiatica. It is tentatively identified and that is why the complete name of the bacteria is pronounced to be Candidatus Liberibacter asiatica The symptoms of the pathogens include yellowing of leaves of certain branches of the citrus plant and once the fruit appears or fruit develops on the tree the fruits are also partially yellow and partially green. They remain partially green even after ripening, and the taste of the fruit is usually bitter. So that once the plant is affected by this particular bacterium the produce of the plant is also not marketable because they are not consumable. The pathogen they survive mostly in the infected Citrus trees and there are certain IPM practices that are recommended that include cultural and chemical measures. Cultural measures include use of pathogen free bud wood for propagation. Since the pathogen is survives in the infected plants so use of pathogen free planting material is the most recommended cultural practice worldwide. Chemicals that are mostly effective is the chemicals that can control the psylids and these psylids are basically responsible for transmission of the disease from one plant to another plant. 500 ppm tetracycline spray fortnightly interval can also have some suppressive effect of the disease on the infected plants. So these control measures, chemicals, along with this cultural measures are recommended for management of this particular pathogen that is Candidatus Liberibacter asiatica for successful management of Greening disease in Citrus. So we have seen that these are the important Citrus diseases and they can be managed successfully if we are able to apply these cultural techniques and chemical techniques in an appropriate way to suppress the pathogens and thereby we can provide good support to the growers for managing these particular diseases.

Thank you.



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