Transmission of Plant Diseases
and most effective control of parasitic plant diseases is possible only if
disease is correctly diagnosed,
nature of transmission of the disease is known and
cycle stages of the involved parasite i.e its mode of reproduction active structures
produced under the favourable condition for repaid and wide dispersal and the structures
produced to overcome adverse condition are known.
study of life cycles of different parasites being beyond the scope of this article it
would be quite sufficient to understand and remember from article\chapter.one of the
general modes of reproduction of different plant parasites viz.fungi, bacteria, viruses
(not true parasites) and nematodes and the structures which enable these parasites to
survive under most unfavourable condition like the off season of suitable host crop(s) and
/ or exceptional high temperature and dry weather conditions prevailing in the summer
months in a tropical country like India.
parasitic as well as viral diseases are transmissible, the parasites or issues being
infectious to suitable host plants with ability to spread from host to host and from one
area to another. The microscopic parasites or sub-microscopic infectious agents viz.
Viruses causing plant diseases are technically termed as pathogens.
disease is established in a particular area or country, transmission of the pathogen from
host to host or from one place to another is termed as dissemination or
dispersal of the pathogen. Dissemination of plant diseases is recognized in
relation to different phases of diseases as under:
- Contact of a pathogen with a suitable host plant and initiation of the
disease first time in the season of a crop is called primary infection. Often
a few or several plants in the crop are likely to get primarily infected.
- : When a plant or few plants are primarily infected, rapid multiplication of the
pathogen sets in under favorable climatic conditions, which helps secondary
spread of the disease.
In case of
fungal diseases the primary infections are usually caused by the resting or dormant
structures of the fungal pathogen while the secondary spread occurs through the asexually
produced, countless and short-lived spores
in majority of plant diseases the above two phases occur sequentially, primary infections
occurring only once in the season and secondary spread often repeating several times in
the same season thus causing rapid. Transmission over wide areas, this type of cycle is
not observed in many other disease. In such cases there is no secondary spread in the same
season i.e. the pathogen multiplies only once during the crop period and the spread of
disease is observed only during the following crop season by way of increased primary
infection. Mildews, leafspots, blasts, blights etc are the common examples of former type
while some smut disease of cereal or grain crops where black powder is formed in place of
grains or inflorescence fall in the latter type. Control measures, therefore, in the
former type need to be directed to avoid the primary infection and also to check the
of primary infections:
to the general mode of primary infection plant diseases are recognized as:-
borne, including diseases carried with planting material.
knowledge is helpful in adopting suitable control measures.
and discontinuous transmission: -
of disease is termed as continuous when it occurs naturally by way of growth,
multiplication and spread of the pathogen in an area or country where the disease is
established. At times, however, in an area or country where a particular disease has never
occurred, it may get introduced through the agency of man carrying diseased material to a
new locality or to a distant country for the purpose of introduction of new plants, crops,
varieties etc. such transmission, of course, is unnatural and regarded as
discontinuous transmission. Altogether different control measures viz.
Quarantine Regulations are enforced to check such discontinuous transmission.
and indirect transmission:-
classifying the methods of disease transmission in relation to the methods of suitable
control measures, the following two groups can be conveniently recognized.
- - Disease transmission where the pathogen is carried externally or
internally on the seed or planting material like cuttings, sets, tubers, bulbs etc.
- The pathogen spreading itself by way of its persistent growth or
certain structures of the pathogen carried independently by natural agencies like wind,
water, animals, insects, mites, nematodes, birds etc. are the different methods of
- False smut disease as well as
Helminthosporin Blight disease of wheat are the common examples of fungal diseases carried
internally through apparently healthy seed. Ring rot and Brown rot of potato caused by
bacteria are carried internally through the tubers. The well known whip smut and red rot
of sugarcane are fungal diseases carried internally in the planting sets. Mosaic and leaf
roll of potato which are viral diseases are also carried inside the infected tubers.
transmission through seed or planting material:-
- In this mode of transmission the
pathogen is carried externally over the surface of seed or vegetatively propagated plant
parts like sets, tubers, bulbs etc. or may even be carried as a physical mixture of fungal
structures with the seed. The common grain smut of jowar is an example of the former type
while the fungal structures called sclerotia having the size of a grain or
slightly bigger in case of the Ergot disease of bajra are often likely to be transmitted
in the form of physical mixture with the seed.
transmission through seed or planting material:-
- It takes place by continuous and persistent growth of the threads or
hyphae of the causal fungi in soil, characteristic of several wood rotting
fungi attacking forest trees and some fruit plants. Some root rotting fungi infecting
certain seasonal crops also are transmitted by this method. The autonomous dispersal of
such soil fungi may range from few cm. To several (8 to 10) meters in a single season.
Some plant parasitic nematodes also exhibit active but limited mobility in the soil.
- Fungal spores produced externally on host surfaces are most easily
carried by wind currents and this is the most dangerous mode of transmission of plant
pathogenic fungi like those causing powdery and downy mildews, leaf spots, blasts, blights
and rust diseases. The black stem rust disease of wheat in India perpetuates on wild
grasses in the Nilgiri hills in the south India from where the rust spores are carried to
south, central & then to north India by wind currents every year. Spores may be
carried from low to very high altitudes of 12,000 to 14,000 feet and from short distances
to very long distances of several hundred kilometers.
and severe epidemics of plant diseases are mostly the results of wind transmission of the
pathogens. Wind dissemination involves four stages relating to the spores viz. Production
of countless spores, their liberation in the wind currents, dispersal alongwith the wind
and deposition on new susceptible host surfaces where they cause infection under
favourable climatic conditions. Apart from spores, bits of fungal threads and nematode
cysts are also amenable to wind transmission in certain cases.
- Disease transmission through the agency of water in different ways is
comparatively less important as compared to the wind transmission. Splashing rain drops
mostly transmit the foliar diseases from leaf to leaf, from shoot to shoot and even from
plant to plant in case of closely spaced crops. Such transmission is usually accompanied
by wind dispersal as well. Plant pathogens requiring high humidity conditions like the
fungi causing downy mildew diseases or bacteria causing canker of citrus are well adapted
to this kind of short distance water dispersal.
soil inhabiting pathogenic fungi and bacteria causing root and collar rots, wilts, foot,
rots, etc are likely to be transmitted to much longer distances through the agencies like
irrigation water, streams and rivers, etc. It is also an important agency in transmission
of seeds of higher flowering parasites like dodder and striga.
Farm animals serve as disease transmitting agents in some cases. They are likely to carry
the pathogen externally on their body surface, particularly on legs and hoofs, etc. or
internally through their intestinal tract. Commonly, the soil inhabiting fungi causing
rots and wilts are carried externally while certain smut fungi causing diseases to grain
crops are transmitted through the intestinal tract.
Although birds play a very minor role in disease transmission, in cases of dispersal of
seeds of higher flowering parasite. Loranthus sp. Parasitising certain trees like mango,
etc. their role is of great significance. They transmit loranthus both externally and
- Farm implements used for cultivation of soil are often likely to transmit
plant pathogens from one place to another. The pathogens in this case are usually carried
in the form of bits of plant disease debris lying in the soil. Similarly tools used for
carrying out operations like cutting, pruning, budding, grafting, thinning,etc. also help
in the transmission of certain diseases from plant to plant. Several viral diseases are
disseminated through the budding and grafting operations.
Most of the viral diseases of plants are transmitted through the agency of different
insects. Both types of insects viz. sucking and chewing or/biting are capable of
transmitting viral diseases. The transmission may be simply `mechanical or it may be
`biological. In the latter case the specific insect and the specific viral pathogen
have some kind of association or relationship between the two. Insects in such cases are
called the `vectors for the particular viral pathogen. In case of mechanical
transmission the pathogen is simply carried externally or internally by the insect.
carried `biologically by the insect vectors are of two types:
pathogen requiring no latent or incubation period in the insect body.
viral pathogens requiring certain incubation period inside the vector body before they are
inoculated or transmitted to healthy host.The insects responsible for transmission of
viral diseases belong to the species of aphids, jassids (leaf hoppers), white flies, mealy
bugs, etc. Certain bacterial and several fungal pathogens are also known to be carried by
Mites in contrast to insects are wingless anthropods resembling ticks and having four
pairs of legs and no antennae. It is suspected that some viral diseases of chillies,
tomato, brinjal, etc. have vector relationship with mites.
Nematodes have been observed to transmit viral, bacterial and fungal plant diseases.
Nematodes feeding externally on host plant roots cause injuries to roots which become the
avenues for entrance of fungal and bacterial pathogens infecting plant roots. The Fan-leaf
virus of grapevine is a well known example of transmission through a species of nematodes.
- : Dodder which is higher flowering parasite is known to transmit certain
viral diseases which remain `persistent in the dodder plant. The flowering parasite
after acquiring the virus from infected plant does not show any symptom itself but remains
capable of transmitting the virus to healthy hosts.
- : Man is often responsible for transmission of plant diseases in two ways
handling seedlings, other planting material or fruits are likely to get personally in
contact with plant pathogens like fungi or bacteria. While handling the diseased material
and unknowingly and indirectly transmit the pathogens to healthy seedlings or plant parts
through his contaminated hands. This is a kind of `continuous mode of transmission.
other or`discontinous mode of transmission for which only man is responsible is the
most efficient and equally dangerous phenomenon of transmission of plant diseases between
distant geographical areas often separated by physical barriers like oceans, mountains or
deserts, etc. Such long distances transmission of a disease to an area or country hitherto
free from the disease is usually accomplished by the transport of infected seed, nursery
stock or timber, etc. Thus it is a kind of direct transmission through propagating
history of plant pathology this mode of transmission has often resulted in to some of the
worst plant disease epidemics in new areas, the local host plant stock being not adapted
to the pathogen and consequently remaining highly susceptible. Fungal diseases viz. Late
blight of potato and downy mildew of grapes, Bunchy top of banana, which is a viral
disease and Bacterial blight of paddy are the examples of some severe and important
diseases introducing in India from other countries.
all the countries of the world have suffered from such introduction of new plant diseases
through the agency of man. Concerned international authorities, therefore, are engaged in
promoting studies in this respect and designing regulations to reduce the danger.