Although some microbes are beneficial for human welfare, some of them are used for the production of bio-fertilizer, some are useful for industries, yet there are several microorganisms, which are the cause of plant disease. Such as some viruses, bacteria, mycoplasma, and fungi can cause several types of disease in plants. Some of the viral, bacterial, mycoplasmal, and fungal diseases are described here.

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Viral diseases in Plants

Leafroll disease of potato

Causal organism (pathogen) – Leafroll virus is also known as potato virus I or Solanum virus 14.

Symptom – The first symptom of this disease is the rolling of the leaf margin. The plant may have a shorter inter-node than normal size which results in dwarfism.

Transmission and damage – Transmission takes place through the infected seed tuber and also by the insect vector. The main insect vector is Myzus persica.

Leaf curl of papaya

Causal organism – Tobacco virus-16 or Nicotiana virus-10.

Symptom – Leaf curling, vein clearing, and size reduction are the main symptoms of the disease.

Transmission – No mechanical transmission occurs during grafting, the main insect vector is whitefly (Bemisia tabaci).

Yellow vein mosaic of bhindi

Causal organism – Yellow Vein Mosaic Virus (YVMV) or Hibiscus virus-I.

Symptom – Main symptom disease is vein clearing, followed by veinal chlorosis of leaves.

Transmission – The virus is transmitted in nature by whiteflies (Bemisia tabaci) and perhaps also by bhindi leaf-hopper (Empoasca devastans).

Bacterial Diseases in Plants

Citrus canker

Causal organism – Xanthomonas citri

Symptom – The disease affects mostly leaves, twigs, thorn, and fruits, but also affect all above the ground parts of the plant. Round, raised, water-soaked brown translucent spots appear on the leaf.

Transmission – Infected part of the plant is the main source of inoculum. The pathogen enters into the host through the natural opening and wound and mainly disseminated by wind, rain, and insects. The optimum temperature of 25-30°C is favorable for the spread of disease.

Bacterial blight of cotton

This disease is also known as angular leaf spot of cotton or vein blight or black arm disease.

Causal organism – Xanthomonas malvacearum.

Symptom – Mainly aerial parts are infected, earlier symptom is water-soaked lesions on the lower surface of cotyledons. On leaves symptom appears as angular spots, most of the branches become black.

Transmission – Transmission takes place through seed so primary infection is through seeds. This bacterium can remain viable for up to six months in soil.

Bacterial blight of paddy

Causal organism – Xanthomonas oryzae.

Symptom – Symptom mainly appears on the leaf in August month. Symptoms mainly appear on leaf sheath and leaf blade, like water-soaked translucent spots. If infections are severe grains are also affected.

Transmission – Main sources of the primary infection are disease stubbles and seeds, and secondary infection takes place through wound and stomata.

Mycoplasmal disease in Plants

Little leaf of brinjal

Causal organism – Causal organism is mycoplasma, which is spherical to oval in shape.

Symptom – There are extreme reductions in the size of leaves and nodes of the infected plant. There is smoothening of leaves is takes place, leaves also become narrow, soft, and yellowish.

Transmission – Transmission takes place through graft and also by a vector, Cestius phycitis.

Sandal spike

Causal organism – Disease is caused by the mycoplasma-like body (M.L.B.)

Symptom – The main symptom is the reduction in the size of leaves and internodes of the Sandal plant (Santalum album). Leaves become yellowish and in later stages become red. The internal symptom is phloem necrosis and ultimately plant die.

Transmission – Transmission is through root contact and insect-vector like Moonia albimaculata, Nephotettix virescence.

Fungal diseases in Plants

Wart Disease of potato

Causal organism – Synchytrium endobioticum

Symptom – Symptom mainly appears on the tuber of the potato in the form of dark brown warts. These warts may have a cauliflower-like excrescences appearance. During severity of infection, galls appear on aerial-shoots.

Transmission – Infection in tubers takes place during the spring season. Zoospores enter into the epidermal cell of the tuber via amoeboid way by making pore.

White Rust of crucifers

Causal organism – Albigo candida (Cystopus candidus)

Symptom – Symptom appears aerial parts of the host leaves, stems, flowers, and fruits, in the form of white pustules on the host surface. Infected leaves become dwarf, thick, and fleshy and flowers are sterile.

Transmission – It is soil born disease. Pathogen perpetuates by oospores lying in the remains decaying part of the diseased plant, which germinate and infect new plant during the favorable condition.

Loose Smut of wheat

Causal organism – Erysiphe graminis

Symptom – Symptom mainly appears on the ears of the infected plant, every ear of the infected plant is converted into a black mass of spores and there is no grain formation.

Transmission – Chlamydospores are carried to the healthy plants by wind, and grow on new plant and produce new mycelium

Late Blight of potato

Causal organism – Phytopthora infestans

Symptom – Symptom appears on any part of plant petioles leaves and tubers. On the aerial parts of the plant, the symptom appears irregular water-soaked areas. On the tuber, two types of symptoms appear wet rot and dry rot. If the atmospheric condition is wet cells of tuber become soft and dark brown called wet rot. If the atmospheric condition is dry then the anterior part of the tuber became black.

Transmission – The main source of transmission is the infected tuber. Dormant mycelium in the tuber grows during favorable conditions.

Tikka disease of groundnut

Causal organism – Cercospora personata & Cercosporum arachidicola both mostly occurs on the same twig and Mycosphaerella berkeleyii and M. arachidicola.

Symptom – Symptom produce by both fungi differ in shape and size and color. All aerial parts of the plant may be attacked by fungus. Lesions are surrounded by yellow halos in letter stages. On the upper surface of leaves necrotic area reddish-black, and at the lower surface light brown.

Transmission – Infection is through conidia on the debris of the diseased plant.

Red Rot of sugarcane

Causal organism- Colletotrichum falcatum

Symptoms- Symptom appears on the aerial part of the plant. During early infection, leaves become yellow and drop from the plant. On the stem, symptoms appear as length-wise split and red blotches. In later condition center of lesions become straw-colored.

Transmission- Main cause of infection is the cultivation of diseased cane. The secondary infection is caused by conidia.

Early Blight of potato

Causal organism- Alternaria solani

Symptom- Symptom appears on a 3-4 week old plant. Yellowish-brown spots appear on the leaves, which enlarge to form concentric rings. Infection in tuber cause change in color of pulp to brown and texture become corky.

Transmission- The primary cause of infection is mycelium and conidia present in the dead and decaying parts of the plant that remain in the soil.

Black rust of wheat

Causal organism- Puccinia graminis

Symptom- Symptom mainly appears in January and March. On the aerial part of the plant black rust appear

Transmission- This pathogen has an intermediate host Berberis leaf. The life cycle is completed in five stages

• Uredospore stage

• Teleutospore stage

• Basidiospore stage

• Pycnidiospore stage

• Aeciospores stage

Among these 1-3 stages are on the primary host (Triticum aestivum) and 4-5 stages are the alternate host (Beberis vulgaris).