Passive or Indirect dispersal:

Passive dispersal of plant pathogens happens through animate and inanimate agents.

1) Animate agents:

a) Insects:

• Insects carry plant pathogens either externally (epizoic) or internally (endozoic).
• They can disseminate bacteria, fungi, viruses, mycoplasmas, spiroplasmas, rickettsia, etc.

Fungal diseases:

• The spermatial oozings at the mouth of spermagonia in the ascomycetes attract various type of insects, flies, pollinating bees and wasps which play a dual role, viz., pollination and transmission of plant pathogens.

Bacterial diseases:

• The fire blight organism (Erwinia amylovora) and citrus canker bacterium (Xanthomonas axonopodis pv. citri) are transmitted by flies (bees) and ants and the later by leaf miner respectively.

Viral diseases:

• Both Aphids (Aphididae) and leaf hoppers (Cicadellidae or Jassidae) in the order Homoptera contain largest number and the most important insect vectors of plant viruses.
• Certain species of mealy bugs and scale insects (Coccoidae), whiteflies (Aleurodidae) and tree hoppers (Membracidae) in Homoptera also transmit virus diseases.

Virus-transmitted diseases and their vector

Mycoplasma diseases: Plant MLO’s are phloem inhabitants and those insects which are feeding on phloem of plants transfer the MLO’s. Mycoplasmal diseases are mostly transmitted by leaf hoppers.

 Ex: Sesamum phyllody (Orosious albicinctus) and little leaf of brinjal (Hishimonas phycitis)

b) Mites:

• Mites belonging to the families Eryophyiidae (eryophyiid mite) and Tetranychidae (spider mite) of class Arachnida transmit plant viruses.
• The genera Abacarus, Aceria, Eriophyes and Brevipalpus are important.

Ex: Aceria cajani transmits Pigeonpea sterility mosaic virus . Aceria tulipae transmits wheat streak mosaic.

c) Fungi:

• Some soil borne fungal plant pathogens carry plant viruses in or on their resting spores and zoospores, and transmit them to susceptible hosts during the infection process.
• Many soil borne viruses are transmitted by the members of Chytridiales and Plasmodiophorales.

Fungal transmitted viruses

d) Nematodes: Several nematodes act as vectors for transmission of fungi, bacteria and viruses.

Bacterial diseases:

• The bacterium which causes yellow ear rot of wheat (Corynebacterium tritici or Clavibacter tritici) is disseminated by ear cockle nematode, Anguina tritici.
• If these two diseases appear together, a complex disease called tundu of wheat occurs.
• Corynebacterium tritici is not capable of dispersal and infection unless it is carried by Anguina tritici.

Fungal diseases: Similarly, root rot and wilt pathogens such as Phytophthora, Fusarium, Rhizoctonia, Verticillium, etc., are disseminated by nematodes.

Viral diseases:

• Many soil borne viruses are known to be transmitted by the nematodes.
• Xiphenema, Longidorous, Trichodorus and Paratrichodorus are the nematode genera belonging to Dorylaimoidea which are known to transmit plant viruses.

The nematode transmitted viruses are divided into two groups on the basis of shape of their particles:

a) nematode transmitted polyhedral viruses (NEPO) and

b) nematode transmitted tubular (NETU) viruses.

Nematode transmitted viruses:

e) Human beings:

Transportation of seeds (seed trade):

• The import and export of contaminated seeds lead to movement of pathogens from one country to another.
• Ex: Late blight of potato, Downy mildew of grapevine, Citrus canker,Fusarium wilt of banana, etc

Planting diseased seed materials:

Planting diseased bulbs, bulbils, corms, tubers, etc.  of vegetatively propagated plants such as potato,  banana, many ornamentals and fruit trees etc., help in dispersal of pathogens from field to field, locality to locality or from one country to another.

During adoption of normal farming practices:

Human beings engaged in preparatory cultivation, planting, irrigation, weeding, pruning etc., help in dispersal of plant pathogens.

By use of contaminated implements:

Pathogens are transferred from one area to another through implements used in various cultural operations (weeding, thinning, hoeing etc.) in the field.

By use of diseased grafting and budding material:

Grafting and budding between healthy and diseased plants is the most effective method of distribution of pathogens of horticultural crops.

f) Dispersal by phanerogamic parasites:

• Phanerogamic parasites transmit the viruses by acting as a bridge between the diseased and healthy plants.
• Cuscuta subinclusa – Cucumber mosaic virus

• Cuscuta california – Tobacco mosaic virus , Tobacco rattle virus , Tomato spotted wilt virus

• Cuscuta campestris – Tomato bushy stunt virus

g) Dispersal by birds:

• In tropics, crows feeding on the fleshy, Sticky and gelatinous berries of gaint mistletoe (Dendrophthoe sp.) deposit the seeds on the other trees with excreta.
• Seeds of Loranthus are disseminated by birds by sticking on their beaks and also through excreta.

h) Farm and wild animals:

• soil inhabiting fungi especially sclerotia adhere to the hoofs and legs of animals and get transported to other places.
• dung having spores or oospores or sclerotia when used as manure spread in the field and act as source of inoculum.

2) Inanimate agents:

a) Wind:

• The dispersal of pathogens by wind is known as anemochory.
• Wind acts as a potent carrier of propagules of fungi, bacteria and viruses.

Fungi:

• Usually the fungal pathogens are light in weight and are well adapted to wind

dispersal.

• Ex: Powdery mildew, downy mildew, rusts, smuts etc.

Both short and long distance dissemination is possible by means of wind.

i) Spores adopted for short distance dissemination- sporangia of downy mildew fungi, conidia of powdery mildew fungi and basidiospores of rust fungi.

ii) Spores adapted to long distance dispersal –uredospores of rust fungi Chlamydospores of smut fungi and conidia of Alternaria, Helminthosporium and Pyricularia.

Dispersal distance:

• If the uredospores reach an altitude of 5000 feet, their distance dispersal in a 30 mile per hour wind could be about 1100 miles, without loosing viability.

Nematodes:

• Ex: Cysts of the nematode Heterodera major, which causes molya disease of wheat and barley, are carried by dust storms from Rajasthan to Haryana

Bacteria:

• Some pathogenic bacteria are carried along with the infected material to short distances by wind.
• Ex: Erwinia amylovora, the causal agent of fire blight of apple and pear, produces fine strands of dried bacterial exudates which may be broken off and are transmitted by wind.

Viruses and phytoplasmas are not directly transmitted by wind, but the insect and mite vectors that carry the viruses move to different directions and distances based on the direction and speed of the air.

b) Water:

• Transmission of plant pathogens by water is called as hydrochory.
• Water dissemination occurs mainly through surface running water and rain splash.
• The mycelial fragments, spores or sclerotia of fungi, Colletotrichum falcatum (red rot of sugaecane), Fusarium, Ganoderma, Macrophomina, Pythium, Phytophthora, Sclerotium, etc., are transmitted through rain or irrigation water.
• Dissemination by rain splash is also called as splash dispersal.
• Ex: Bacterial leaf spot of rice (Xanthomonas campestris pv. oryzae), Bacterial leaf streak of rice (Xanthomonas campestris pv. oryzicola), Green ear of bajra (Sclerospora graminicola).
• Water not only plays an important role in the dissemination of plant pathogens, but also helps in the growth and spore discharge of many fungi.