A Study of Some Aspects of Cercospora Leafspot on Groundnut.

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Gaafar Ibrahim Mohamed Ali, Ali
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Differentiation between Cercospora personata (Berk et. Curt) Ell et Ever, and Cercospora arachidicola Hori, causing groundnut leaf spot disease was made, based on variation in lesion characteristics, mode of sporulation, shape and size of conidia and conidiophores. The lesion size was found to be a variable character and therefore not useful in identification. Three unidentified leaf spots that are commonly associated with groundnut crop in the field and which might easily be mistaken for Cercospora leaf spot were described in detail. Germination of conidia of both Cercospora spp. was tested in water films, on fresh leaf surfaces and in artificial media. Fresh considia germinated readily while conidia stored for 6-9 months under room temperature apparently lost viability because they failed to germinate in all substrates tested. Successful isolation of the two pathogens, from fresh conidia, was made using the single spore isolation technique. Development of the cultures was followed for three months. C. arachidicola grew faster, giving colonies 12mm. in diameter in three months whereas C. personata gave colonies 4mm. in diameter. Microscopic examination of the developing mycelium in young cultures of both species was made. Young cells at the hyphal tips were rod shaped, the intermediate cells were bell shaped, whereas the basal cells were rounded and thick walled. These observations were recorded for the first time. Infection of groundnut (var. Barberton) was produced in the glass-house by spraying four weeks old plants with spore suspension prepared from two weeks old colonies (.5cm in diameter) of C. arachidicola grown in Czapek Dox yeast extract medium. The effect of environmental factors on the disease was studied with special reference to: i) the effect of total annual rainfall on natural infection in season 1967/68, ii) the effect of the monthly rainfall and iii) the effect of daily variations in R.H. in artificially inoculated plants in seasons 1967/68 to season 1969/70 inclusive. Increase in total annual rainfall increased the natural infection from zero percent to 19 and 46 percent at Wad Medani and Abu Naama respectively. However, two localities, Sennar and Abu Naama having equal total annual rainfall gave different levels of infection. The higher level of infections 19-56 per cent at Abu Naama compared with no disease at Sennar was attributed partly to availability of sufficient inoculum from the rain grown crop and partly to the effect of higher rains in June, just before sowing at Abu Naama. The monthly distribution of rainfall had a greater effect on disease than the total annual rainfall. Season 1967/68 which had the highest level of infection. The following two seasons had a low rainfall either in July or August. The level of infection was much lower in both seasons although in season 1968/69 July rains were higher than that of season 1969/70 and in season 1969/70 August rains were close to that of season 1967/68. It is considered that shifting the sowing date of groundnut (var. Barberton) in the Gezira from the beginning of July to the beginning of August may help in avoiding the disease. A high level of R.H. persisting for shorter periods during the growing season. Four levels of infection were produced following artificial inoculation with infected debris suspension and protection with a chemical Shell Matin. The effect of the disease on nut and hay (measured by leaf shedding) yields was assessed in the above treatments. Delayed infection of three months old plants increased the leaf shedding at 31.3 and 62.8 per cent level of infection but had no effect on yield. Lower levels of infection were without effect on all attributes of yield. It was suggested that the nut yield was not effected because the majority of the nuts had already developed before infection became high. Sixteen spray fungicides were tested for the control of the disease in two seasons at the Gezira Research Station. Infection was produced by inoculation of three weeks old plants with infected debris suspension. The fungicides were applied three times starting when the disease was first seen. The effect of the fungicides on the disease was assessed and the increase in the yield of pods and nuts at harvest was determined. Some fungicides reduced the level of infection without showing a significant effect on the fungicides tested, namely Duter, Shell Matin, Dithane M45 and Manzate were found to be the most promising of the sixteen fungicides tested. The best fungicides increased the pod yield by 70-100kg. per feddan and the nut yield by 50-70kg. per feddan.
A Study of Some Aspects of Cercospora Leafspot on Groundnut.