نظرة عامة على إرشادات المسح لبكتيريا Candidatus liberibacter asiaticus المسببة لمرض اخضرار الحمضيات

This video is developed in the framework of the Africa Phytosanitary Program or APP as part of training material to provide guidance on conducting surveys for Candidatus Liberibacter Asiaticus. Candidatus Liberibacter Asiaticus is one of the most serious insect-vectored pathogens of citrus. Once tree is infected, there is no cure. This video will show you how to look for the bacterium in citrus trees and how to collect plant tissue sample for molecular diagnostic identification. This video supplements the complete survey guidance provided by the International Plant Protection Convention and the United States Department of Agriculture or USDA. Let's get started. This bacterium is serious threat to all citrus species, cultivars, and hybrids, and some relatives. The latency period for development of symptoms can vary from 4 to 12 months or longer, which increases the difficulty of detection. Here we have citrus plant showing signs and symptoms of the bacterium. You will observe asymmetric blotchy leaf mottling, which is the most classic symptom, and observed on both sides of the leaf, crossing leaf veins, but will not cross the leaf midrib. You may also observe yellowing of veins, which can become enlarged or appear corky. Thick and leathery leaves can be observed as result of starch accumulation. There are many other symptoms you may observe as the disease progresses, such as yellowing of leaves and leaves that are small and sparse. Appearance of zinc deficiency symptoms including green veins with chlorotic intervenal areas. Green areas on the leaf reduced to small circular spots referred to as green islands. Unlike foliage, symptoms on fruit are much more characteristic of and specific to this disease and include abnormal coloring of fruit, misshaped or smaller fruit, fruit that has bitter taste. In contrast, fruit with similar symptoms caused by other citrus diseases is generally sweeter than normal. When inspecting host trees, examine newer foliage and branches on larger trees. Older foliage is frequently damaged by other pathogens and may make it difficult to see symptoms. You may also notice look-alike signs and symptoms that can be easily mistaken for the bacterium. These can include citrus stubborn disease, Spiroplasma citri, Citrus tristeza virus or CTV, Phytophthora infection, citrus blight, and certain nutrient deficiencies such as zinc, iron, and manganese. Because distribution of the bacterium within the plant can be uneven, it is important to section each tree into quadrants and take some leaves from each quadrant. Collect leaves with midribs that are attached to stems. Stems should be 6 to 8 in long with approximately 15 leaves and petioles attached per symptomatic tree. Some asymptomatic leaves on the same branches may also be submitted with symptomatic ones. Quality samples are needed for diagnostic testing to confirm presence of the pathogen. You will need disposable nitrile gloves, resealable plastic bags, dry paper towels, and way to label the samples. Place leaf and stem samples between dry paper towels in resealable plastic bag per tree. Express air from the bag and seal the bag. Place the bag inside another larger bag. Record sample identification number or sample label on bag. Keep samples cool but not frozen. Disinfect sample collection equipment and wear gloves before and after sampling each host. Molecular methods are needed for accurate diagnostics. Suspect leaf and stem samples based on symptoms should be sent for molecular confirmation. Fruit from infected plants will not contain detectable amounts of the bacterium. Therefore, fruit should not be sent for testing. Before leaving the site, be sure to enter all of the trapping data into your mobile device. For step-by-step instructions to enter survey data, please refer to the video titled Mobile Data Collection Workflow, available on the APP landing page under training. Thank you for taking the time to view this instructional video. This video supplements the complete survey protocols provided by IPPC and USDA. Please refer to the protocols for detailed guidance.