Solanaceae plants have strong allelopathic potential, and therefore the action is confirmed through: a) bioassays with liquid or various solvent extracts and residues, b) fractionation, identification, and quantification of causative allelochemicals. Most assessments of allelopathy involve bioassays of plant or soil extracts, leachates, fractions, and residues which support seed germination and seedling growth in laboratory and greenhouse experiments. Plant growth is also stimulated below the allelopathic threshold, however severe growth reductions may be observed above the threshold concentration depending on the sensitivity of the receiving species. Generally, seedling growth is more sensitive than germination, particularly root growth. Some approaches showed that field soil collected beneath donor plants significantly reduced or somewhat promoted the growth of the recipients plants. Petri dish bioassays with aqueous extracts of different parts of donor plants showed considerable phytotoxic activities in a concentration-dependent manner with leaf aqueous extracts being most dominant. Delayed seed germination and slow root growth attributable to the extracts may be baffled with diffusion effects on the rate of imbibition, delayed initiation of germination, and particularly cell elongation; the main factor that is responsible for affecting root growth before and after the tip penetrates the testa. Light and electron microscopy extract analysis at the ultrastructural level are correctly investigated. Several Solanaceae plants have allelopathic potential, and therefore the activities, kinds and quantity of allelopathic compounds differ depending on the plant species. The incorporation of allelopathic substances into agricultural management might scale back the development of pesticides and reduce environmental deterioration.

Early blight disease caused by Alternaria sp. is one of the most devastating diseases of

Solanaceous crops widely distributed in Sudan. The aim of this study was to determine the

genetic variation among different Alternaria isolates recovered from different Solanaceae

crops showing typical symptoms of early blight disease. Infected leaves of tomato, potato,

eggplant and pepper were collected from different geographical zones in Sudan. The recovered

fungal isolates were identified to the genus level based on cultural and morphological

characteristics. Five representative isolates were sent to the CABI Bioscience, U.K. for confirmation.

The genetic relationship among the isolates was determined using the amplified

fragments length polymorphism (AFLP) technique and the generated data were used to

create similarity matrices using the PAST 3.01 software package. Dendrograms were constructed

based on Jaccard’s similarity coefficients. A total of 70 fungal isolates was recovered

from the tested plants and all of them showed morphological characteristics typical

of Alternaria spp. The conidia appeared in multiple-branched chains with spore sizes in

the range of 2.38−13.09 μm × 12.30−43.63 μm. Therefore, the isolates were identified as

Alternaria alternata (Fr.) Keissl. The identification was then confirmed by CABI.AFLPbased

dendrogram which revealed five clusters with a significant cophenetic correlation

coefficient (r = 0.834) between the dendrogram and the original similarity matrix irrespective

of their geographical origins. Eighteen (75%) of the Alternaria isolated from tomato

leaves were clustered together in cluster I and five isolates formed two separate clusters,

viz. cluster IV (T-Kh5 and T-H1) and cluster V (T-H4 and T-Med2). The remaining isolate,

T-Am5, grouped with one of the potato isolates in cluster III. The other isolates which were

recovered from potato, pepper and eggplants were all separated from the tomato isolates

in the largest cluster.