The late blight pathogen is still the cause of considerable harvest losses in many regions of the world. Results from field studies show that epidemics typically cause yield losses of between 40% and 70%, depending on varietal susceptibility and environmental conditions. If infection occurs early in the season, the entire harvest can be lost.As well as reducing yield, the pathogen also causes reductions in quality, which can bring considerable economic penalties.
Sources of infection
The fungus Phytophthora infestans can spread in two ways: as asexual vegetative mycelium in infected tubers,or as the sexual stage, in the form of resting spores, the so-called oospores. Sexual reproduction requires the presence of two different mating-types.
Sexual reproduction is required for oospore formation. These resting spores are probably important for the long-term survival of the pathogen in soil. However, their importance in the infection cycle of thepathogen has not yet been finally determined.
Transmission of the pathogen
Phytophthora infestans as vegetative mycelium is only possible via infected plant parts. This means that the fungus can only survive in tubers that are not killed off by frost during the winter. Phytophthora can overwinter in tubers via three routes:
• potatoes on cull piles
• volunteer potatoes
• seed potatoes
Biological relationships
Tubers can become infected while they are still growing, or later, during harvesting operations. If infected tubers are then planted out in the following spring, the pathogen’s mycelium grows intercellularly through the tuber tissues, entering young sprouts as the tuber germinates, and is then carried upwards in a latent form within the shoots. Another route of infection occurs when the fungus sporulates on infected tubers, and the spores thus released succeed in infecting the lower leaves and stem parts. A further route of transmission occurs when spores spread between tubers in soils with high water content.After infection, the further development of the fungus is predominantly determined by climatic conditions. Sporangium formation requires high humidity, and has a temperature optimum in the range 18°-23°C.Under optimal conditions, a successful infection
canusually occur within 2 hours. The infection process can take place on either side of the leaf.
The incubation period, i.e. the period between penetration of the host and the
first appearance of lesions, lasts 2-3 days.
The infection cycle continues when the pathogen sporulates. Sporangia are released into dew or raindrops and can enter the upper soil layers when this water runs off the leaf. Here, they release their zoospores, which are able to penetrate into very young tubers through the epidermis.
The pathogen has a further opportunity to infect tubers during the harvesting process, during which tubers come into contact with infected foliage or with earth contaminated with sporangia. Inoculum in the form of sporangia can remain viable in soil for some time (ca. 30 days). Even small injuries to tubers are sufficient to allow infection.
Symptoms
Because potato leaves tend to develop various brown spots during the vegetative period, it is important that advisors and farmers are able to identify late blight symptoms clearly.
Primary infections are seen on the stem and the petioles (leaf-stalks). These plant parts become brown, and eventually nearly black, and the associated leaves die off.
Surface growth of the fungus is rarely seen on stems.
Initial symptoms of leaf infection are small, yellowish to dark-green spots.
Infection usually first occurs on leaf margins and leaf tips, because this is where water droplets are retained longest. Under favourable environmental conditions, the spots quickly enlarge and become dark brown to black. A whitish-grey zone of
downy fungal growth can be seen on the underside of the leaf at the border between infected and healthy tissue. This is particularly obvious to the eye during periods of high air humidity, in the early morning (dew period) or after rainfall. The white zone is the unmistakable sign of late blight infection.
Infected tubers have more-or-less large,irregular and slightly sunken, blue-grey spots on their surfaces. Below the areas showing these symptoms, large parts of the starchy tissues are discoloured rustybrown;there is no sharp distinction between the rusty-brown areas and healthy tuber tissue .
Implications for agricultural practice
The results of tests of the effectiveness of curative treatments show that it is almost impossible to prevent the outbreak of an epidemic once infection has become successfully established. Therefore, it is important that the establishment of the pathogen is avoided through preventative measures. A pre-condition for this is the correct timing of the start of the spraying programme, before the first symptoms appear. Modern communications media provide the basis for the rapid translation of decision into action.
The integration of the latest research results coming from the areas of phyto pathology,plant breeding and crop protection will provide the basis for the successful control of the pathogen Phytophthora infestans into the future.
(Compiled and written by Harsh saxena)
