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Abstract
SUMMARY
The aim of this investigation was to isolate, identify and study the pathological properties of some cucurbit seed-borne fungi. One of the important goals of this study was to check the efficiency of some fungal and bacterial biocontrol agents on controlling damping-off disease incited by the tested pathogens both in laboratory and under greenhouse conditions. Comparative study was carried out to test the efficiency of some chemical fungicides on disease control.
The results of this investigation could be summarized in the following items:
A. Isolation and Pathogenicity experiments
A.1. Isolation experiments
1. Isolation was carried out from seeds of cucumber, watermelon, melon, squash and pumpkin. Samples were collected from 3 Egyptian governorates (Alexandria, El-Bohayra and Assiut) and 2 Libyan governorates (Tripoli and Misurata). Isolation and identification studies revealed many fungal isolates namely, F. oxysporum, F. semitectum, F. moniliforme, F. solani, M. phaseolina, R. solani, A. alternata, S. sclerotiorum, Stemphylium sp., Nigrospora sp., P. ultimum, Aspergillus sp., As. flavus, As. niger, As. ochracus and Penicillium sp. Frequency of the isolated fungi was determined and results revealed that:
• Total No. of isolates was higher in the Egyptian governorate El-Bohayra and of Libyan governorate Misurata, especially those isolated from squash and cucumber (24.88 and 30% of total No. of isolates, and those isolated using agar technique (30.4 and 47.88% more than those isolated by blotter technique).
• The most frequently isolated fungi included F. solani, R. solani, and M. phaseolina (included in the 1st group > 10%). Stemphylium sp., As. ochracus and F. moniliforme were the least frequent (included in the 4th group < 4%).
• Total isolates isolated from the external seed coat were 2.3–2.65 times more than those isolated from internal seed parts. R. solani, F. solani and M. phaseolina were the most frequent among other fungi isolated from seed coats (1st group > 10%). S. sclerotiorum was isolated only from squash and pumpkin seeds, whereas P. ultimum was isolated only from cucumber seeds.
2. All the tested fungi were able to induce variable pre-emergence and post-emergence damping-off symptoms as follows:
• The highest total infection percentages were produced by R. solani, S. sclerotiorum, P. ultimum and F. solani. The percentages were higher in Thamin cucumber cultivar than Alpha cv. and in Tezier squash cv. than Eskandarani squash cv. (1.85 times more).
• All the tested isolates induced pre-emergence damping-off diseases. S. sclerotiorum was the most virulent among the other tested isolates (54.16% in cucumber and squash, compared with control), followed by P. ultimum in cucumber and M. phaseolina in squash.
• The highest post-emergence damping-off percentages was induced by R. solani in both cucumber and squash (41.66% and 37.5%, respectively). Thamin cucumber cv. and Tezier squash cv. showed more PTD%.
3. Microscopic observations of inoculated seedling sections showed complete destruction of epidermal cells and the outer layers of cortex during the first 24 hours after inoculation with R. solani or F. solani. After 48 hours, complete colonization of cortex occurred inter-, and intracellularly. The fungal hyphae extended deeper in the host. Four days after inoculations, invasion with R. solani extended to the vascular tissues and limited layers of pith, whereas partial invasion of vascular tissues occurred in F. solani.
B. Disease control
B.1. Chemical control
B.1.1. In vitro studies
4. All the tested fungicides significantly reduced the radial growth of the tested damping-off pathogens at the full recommended concentrations. R. solani and S. sclerotiorum were more sensitive to fungicidal treatment than the other tested pathogens, where fungicidal effiency was 66.7%, compared with control; Zineb and Topsin were more effective on M. phaseolina, whereas Benlate and Topsin were more efficient in suppression of R. solani hyphal growth. Alitte was completely inactive on P. ultimum.
5. Inhibition zone technique results confirmed, to great extent, these obtained in linear growth method. Inhibition zones produced by full dose application were higher than those of the half dose. S. sclerotiorum was very sensitive, where inhibition zone was 24.2 mm. Higher IZ values were also obtained by Alitte, Benlate, Dithane and Topsin (19.32-22.32 mm). A. alternata was insensitive to both Topsin and Benlate.
B.1.2. Greenhouse studies
6. Significant reductions in damping-off incidence in both cucumber and squash cultivars were obtained due to treatment with any of the tested fungicides, however, Topsin was more efficient in reducing damping-off incited by R. solani and S. sclerotiorum (72.01% and 79.89%, respectively compared with control). On the other hand, Dithane was more efficient in controlling damping-off caused by P. ultimum, M. phaseolina and F. solani, where reduction%, compared with control, attained 59.99%. Generally, Dithane was more efficient in controlling PRD incidence, whereas Topsin was more efficient against PTD. Where reduction rate was 61.44%.
B.2. Biological control
B.2.1. Bacterial biocontrol agents (BBA)
B.2.1.1. In vitro studies
7. All the tested BBA’s were effective in reducing the radial growth of the tested damping-off pathogens. Moreover S. maltophilia was more effective (77.07%), whereas S. sclerotiorum was the moist sensitive to BBA’s treatments, where reduction rates attained 64.41%. On the contrary, R. solani was the least sensitive to BBA treatments, where reduction rate was 24.03%.
8. The highest IZ values were obtained by the BBA B. subtilis (25.55 mm as a mean values). S. sclerotiorum and M. phaseolina gave the highest IZ values due to BBA treatments (33.00 and 28.66 mm, respectively).
9. All the tested BBA’s treatments significantly reduced TIP incidence caused by the tested cucurbit seed-borne pathogens. Reduction rates attained 62.49% in B. subtilis and St. maltophilia, compared with control. Moreover, reduction rates were higher in R. solani and S. sclerotiorum treatments.