Alternaria brassicae caused black spot disease of Rapeseed-Mustard (Oilseed rape). Long term storage of Alternaria brassicae isolated resulted loss of virulence. Although growth of culture is good on tomato extract agar media. While infecting host it does not showed proper infection.
It is not difficult to prove the pathogenicity of Alternaria. What method of inoculation and spp. of crucifers used resulting in symptomatology. Normaly when we used the crucifers spp. belonging to B. napus, or B. carinata or B. campestris var. Y Sarson etc., we have to use carborundum powder to make injury in waxy layer. Other precautions of proving the pathogenicity like high humidity, low temperature, no direct sunshine etc etc. needs to be taken care
P. D. Meena .
Yes, of course, it is difficult to prove as you said but with the implication of advanced techniques which facilitated the control of this fungus. Since valuable improvement has been made in investigating the molecular evidence for the biosynthesis of phytotoxic secondary metabolites and their role in plant disease development. Good luck
There are several reasons for your observation
1. Mutation: The pathogen may have mutated due to long term storage thereby reducing its level of virulence when inoculated into a susceptible host.
Note: Try using a fresh isolate for the pathogenicity test
2. Method of inoculation: use abrasives for waxy or cutinized surfaces, or sharp objects to mechanically injure the plant prior to inoculation with the spores of the pathogen or spread around the root rhizosphere (for root infections of plants) etc.
Note: Whatever method or strategy you choose to adopt, the pathogen must be aseptically inoculated into the cortex of the host plant.
3. The Age of the host plant: Many pathogens are specific in their nutrient requirements
Example: a pathogen that can cause damping of disease in seedlings of a particular plant might not necessary cause infection in the matured stage of that same plant species.
Note: If your Alternaria spp was isolated from the fruit of matured rapeseed or mustard plant, then pathogenicity test should be conducted using a healthy plant of similar age range
Dear Dr. Peter,
I do agree with the point 1 (mutation) and 2 (inoculation method) but in case of Alternaria brassicae, natural disease ocurs on older lower leaves particulary after 45 days of seeding. We also observed its infection at cotyledonary stage.
Therefore, my question is that the Alternari brassicae cultures are not doing so. Is there are chances of any modification in there nurishment to achive there loosed virulence again?
Thank you so much for your answer
Regards
If somebody proved the pathogenecity of Alternaria brassicae, it is good but the methodology should be used commonly. But it is not so because of notorious pathogen may play different games. Subsequent subculturing makes it sleepy and once infect again the host will increase virulence. But question is that the virulence level may be goverened by genetic character of any organism must be uniform.
Yes, I agree, subsequent subculturing makes Alternaria spp. (including A. brassicae) weakly grow on media, produce spores and lower the virulence. I suggest that you use lyophilization (Freeze drying), this method/technique will give good hyphal growth, spore production and viulence .l
Yes but pathogen can be maintained without loosing their virulence very well on any medium having plant extract or some grain powder like chick pea powder, or brassica seed powder, or whole grain etc and kept at low temperature.
Using fresh culture of Alternaria brassicae is difficult to study of more number of isolates. Maintenance of isolates by subsequent subculture may loose the virulence, deformed conidia, and poor growth of fungi. We used tomato extract media gave better results regarding sporulation, growth and size of conidia etc. But pathogenesis is still problem may be recovered by inoculation on host again and again. Although, host age is certainly influence the disease initiations.
The best way to retain high virulence is the re-isolation of Alternaria from the host, sprayeing the spores at third leaf stage on a high sescebtible host will skip the influence of the host age on disease initiation. As this process required longer time to get a high virulent isolate, I sugest to use the lyophilization (Freeze drying).
Is tomato extract media better than V8 juice? I only used V8 media but the spores production was not consistant.
Unfortunately, V8 juice media is not found in my locality so for my knowledge but I used V8 media in Rothamsted Research, UK for isolation of Alternaria brassicae was really better media for its growth. Synthetic V8 juice media in powdered form is not so suitable so far. I tried once, the methods of preservation for Alternaria brassicae culture but results were not so encouraging.
Successive sub culturing for maintaining the culture leads to loss of virulence. It is common problem for all pathogens. So to prove pathogenesity we should take the fresh culture. Or we can use the process which maintains culture for longer time period, so as to reduce the no. of subculturing and retain the virulence of pathogen.
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