الفهرس يوجد فقط 14 صفحة متاحة للعرض العام
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المستخلص
Onion is considered one of the most important vegetable crops in the world including Egypt. In 2005, several diseases such as stemphylium leaf blight caused by Stemphylium vesicarium attacked onion plants.
Chemical control is considered the most effective means to control this disease. One of the main problems restricting chemical control of fungal diseases is pathogens that could produce resistant strains to the used fungicides, particularly to those used repeatedly at the same site for several subsequent seasons.
The present study aimed to investigate probabilities of generating resistant strains by Stemphylium vesicarium causing stemphylium leaf blight disease. Assessment of resistance levels and studying factors that affect the occurrence and development of acquired resistance were also taken into consideration.
The results obtained from this investigation can be summarized as follows:
1- The stemphylium leaf blight causal pathogen was isolated and identified as Stemphylium vesicarium (wallr.) Simmons and the telemorphic state Pleospora allii (Rabenh) Ces. & De Not.
2- Testing the Pathogenicity of 14 isolates of Stemphylium vesicarium on Giza 6 onion cultivar under greenhouse conditions indicated that all isolates of the pathogen were able to infect onion plants with different degrees of severity causing typical symptoms of stemphylium leaf blight disease.
3- All isolates of Stemphylium vesicarium showed different degrees of resistance to the tested fungicides, (Ridomil mancozeb, mancozeb and copper oxychloride). In addition, the values of EC50 were greatly fluctuated according to the tested isolate and tested fungicide.
4- Isolate A4 showed sector at concentration 400 ppm of fungicide Ridomil mancozeb. This sector was picked up from the culture and exposed to higher concentrations, and proved highly resistant to Ridomil mancozeb than the mother isolate.
5- The sector isolate, showed highly cross-resistance to Ridomil mancozeb and mancozeb, and weak or no cross-resistance to Ridomil mancozeb and copper oxychloride.
6- The resistance levels were negatively related with the increase and decrease in temperature.
7- Two isolates of S. vesicarium had acquired resistance to fungicide Ridomil mancozeb and the reaction varied by the different periods of exposure to UV rays (0, 5, 15 and 45 min). Isolate S1 was more affected than isolate M which exhibited the highest resistance index (10.63) as a result of exposing isolate S1 to UV rays for 15 min. Isolate M irradiated for 15 min. revealed raising in resistance index to 3.55 compared with 1.00 in non irradiated isolate. Whilst, irradiation for 5 and 45 min with the isolate S1 resulted in raising the resistance index to 5.08 and 1.14, respectively. Irradiation of isolate M for 5 and 45 min raised the resistance index to 1.92 and 2.58, respectively. Exposing two isolates of S. vesicarium to UV rays had no significant increase in resistance to the fungicide mancozeb, however, irradiation of isolate S1 for 45 min. resulted in more sensitivity to this fungicide.
8- Positive cross resistance between Ridomil mancozeb and mancozeb was found, while weak cross-resistance between Ridomil mancozeb and copper oxychloride, Mancozeb and copper oxychloride were detected.
9- The resistant isolate (M) was remarkable by a dark red colour cultures with white cottony mycelium and decreasing of conidia production of spores, while sensitive isolate (S1) was distinguished by gray colour cultures and increasing in production of conidio-spores.
10- The activity of polyphenoloxidase, peroxidase and catalase enzymes were estimated in M, A4S, S1 and A4m isolate. Increasing in activity of the three enzymes of isolate M and A4S than S1 and A4m isolate were detected.