Role of Microorganisms in Disease Spreading in Citrus

(By Iqra Nazir*, Mujahid Ali**, Ammara Ainee*)

(*IFSN, UOS; **Horticulture, UOS)

Microorganisms are playing a great role in disease spreading in all organisms. They are destroying our citrus production. Citrus is second most produced fruit crop in the world after grapes. Mandarins (Citrus reticulata) are a very important group of citrus, which is the main fruit crop of Pakistan. Fruit drop is a serious threat to citrus industry of Pakistan. Citrus canker is the disease of citrus. The causal organism is Xanthomonas axenopodis P.V citri. This is a bacterium. There are two types of bacteria Gram +ve and Gram-ve bacteria. Xanthomonas is a Gram-ve bacteria. XAC (Xanthomonas axenopodis P.V Citri) enter through stomata in the leaf canopy when stomata open XAC enter the stomata wound 5 directly expose mesophyll tissues due cultural practices and insects. Go to appropriated sites and organs or tissues. Settle and multiply in the organs (infection). Infection is the establishment of the pathogen into the host. After that symptoms appear in the form of the disease. The latent period is involved in it. The time between inoculation to injection is called latent period. Xanthomonas release chemical is known as xanthomonadin which interrupt the normal physiological mechanism of the plant. In the early-stage attack of XAC is high as compared to a later stage. There are 7 to 10 days for symptom appearance after inoculation. symptoms are pinpoints spots that may reach high up to 2 to 10mm size. spots appear first on lower sides than upper sides. spot size depends on the age of injection, type of cultivar and time of inoculation. Raised pustular with definite margin and sunken center deep from the marginal surface. The sunken center is spongy and corky this condition is known as cratriform. Water soaked margins around necrotic tissues which lead to necrosis (death of tissues by autolysis). The yellow halo around each spot-on leaf. It’s fruit and stem lesions size may be up to 1mm.Its vector is citrus leaf miner which forms the galleries on the leaves expose the mesophyll tissues. The biology of the pathogen is Gram –ve, a rod shape, single polar flagellum. The favorable environment is warm, wet condition. The temperature is minimum 20-degree centigrade, optimum 28 to 30-degree centigrade and the maximum is 39-degree centigrade. The spreading method is when it prevails wet condition bacterial ooze come out from bacterial canker spot. Millions of bacteria are present in ooze. Rain and wind spread the ooze and enter through stomata. The pathogen is persistent that can survive for a long time. This pathogen firstly found in Florida USA in 1912. Then in Japan, Africa, Europe epidemic in 30 countries. Citrus Greening is another disease of citrus. It is the 2nd most important disease of citrus.

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In China it yellow shoot disease while in Africa it is yellow branch disease, yellow dragon disease. First, it is a belief that it is a viral disease but later discover that it is a bacterial disease Not grow on artificial media. It is phloem-limited bacteria. Casual organism candiddatus liberibacter. It is a rod-shaped bacteria. Citrus psyllid is a vector for it. It acquires from infected source then they transmit to the healthy plants. The citrus psyllid is a sucking insect. They suck on the lower side of the leaf. After acquiring the infected source, it feeds on Lowe side of the leaf. During sucking the plant sap bacteria transmitted into the phloem. Settle down in the phloem and multiply. Established in the phloem (infection). Plant defense will activate. Symptoms appear epidemiology is Asian form can tolerant up to 30 degrees centigrade. African and American form heat sensitive up to 20 to 25 degrees centigrade. Symptoms are yellow shoot, phloem-limited, interrupted with photosynthetic. Yellow foliage half of the fruit remain yellow, uneven fruit ripening.

Citrus malanose is also a disease of citrus. Its causal organism is Diaporthe citri. Symptoms are many dark spots of various sizes merge on fruit and leaf surface. This condition is known as mud cake malanose. Spots on leaves and fruit flow with water on the surface. This condition is known as tear stain malanose. Many spots appear close and form the fruit and leaf surface rough. This condition is known as sandpaper malanose. Citrus malanose is a fungal disease. Citrus gummosis is a disease of citrus in which we can see the gum production on root and trunk. The causal organism of this disease is Phytophthora citrophthora. symptoms are the bark of stem and branches are cracked. From cracks, sap oozing occurs this condition is known as bleeding appearance with the passage of time sap sticks on the stem just like gum. Gum girdle the whole trunk. Bark may be removed easily. Leaves show the nutritional deficiency. Disease life cycle can be described as zoospores moves with the irrigation water then enter the root of the plants, germination and gum formation.

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Citrus dieback is a disease of citrus. It is caused by colletotrichum gloeosporidies. Symptoms are it starts drying slowly from top to bottom of branches and twigs. Branches and twigs look like scorched by fire. Minute slightly raised clumps pustules can be seen on affected branches and twigs. Dry twigs look like Ash color.

The blue mold of citrus is caused by Penicillium italicum. Airborne spores that can contaminate and transmit container with movement, packing houses, storage, cleaning equipment and water .it is a post-harvest disease. Symptoms are the blue colored sporulating area on affected fruit. The affected area is surrounding by white mycelium in a narrow range.

The green mold of citrus is caused by Pencilium digitatum. Symptoms are the sporulating area on affected fruit have olive green spores. The affected area is surrounded by broad zone with whitish mycelium. A brush like structure can be seen under microscope known as penicillus brush. Citrus stubborn disease caused by bacteria which is phloem-limited. The causal organism of this disease is Spiroplasma citri. The vector of this leafhopper which is sucking, phloem-limited. Symptoms are an irregularity in fruit arrangement on same plant shape, size, and color. Affected fruit drop out. Irregularity in affected fruit can be seen by cutting into two halves. Bunchy appearance on the top of the affected fruit. There two ends of fruits blossom end green and stem end discolored.

Citrus variegated chlorosis is caused by organism Xylella fastidiosa. It is a xylem limited bacterium. So, it is a bacterial disease. The vector of this disease is sharpshooter leafhopper. it is a sucking insect suck the sap from xylem. Acquisition feeding period is 2 hours. It has a wide host range. It has several generations per year. Symptoms on leaves are interveinal (between the veins) chlorotic lines. Gummy lesions on lower sides of the leaf. Gummy lesions become black and necrotic. Symptoms of fruits look like damage, changed color, the rind is held, lack of juice and acidic flavor. Symptoms of the tree are reducing vigor and growth with abnormal fruit and flower setting. In this way not only, fruit yield but also fruit quality in decreasing in citrus.

Mujahid Ali

I am a Ph.D. scholar in Institute of Horticultural Sciences, UAF and Ex-Visiting Lecturer in Horticulture UOS, Ex-Research Fellow in ACIAR project on vegetables and Erx-Teaching Assitant in Horticulture UAF. Moreover, Ph.D. IRSIP done in the United States.

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