Effect of Nutrition and Physical Factors on Mycelial Growth and Production of Pigments and Nonchromatic UV-absorbing Compounds of Alternaria eichhorniae

Alternaria eichhorniae is a host-specific biocontrol agent for managing waterhyacinth ( Eichhornia crassipes ) in Egypt. An important diagnostic characteristic of this fungus is the production of crimson-red phytotoxic pigments in the medium under certain conditions. A virulent isolate, A. eichhorniae 5 (Ae5), was studied to determine the optimum conditions for its growth and production of pigments and nonchromatic UV-absorbing metabolites (UVACs). The maximum production of pigments was obtained when cultures were grown on potato dextrose agar (PDA) with 20% dextrose, an initial pH of 4.5 at 25–30°C under continuous darkness or diurnal light, and without wrapping the culture plates. The maximum yields of the nonchromatic, 229- and 286-nm-absorbing compounds were obtained on PDA with 20–50% of dextrose at 20–30°C under continuous darkness. Culture plates, unwrapped or wrapped with only one layer of Parafilm at pH from 3.8 to 6.2 were favourable in this respect. There was a strong inverse relationship between linear mycelial growth and pigmentation as a function of light. Furthermore, the reduction in aeration, presumably proportional to the number of layers of Parafilm wrappings, led to lower levels of the red pigment(s) and the nonpigmented UVACs, to reduced mycelial growth, and a suppression of sporulation. Different culture-filtrate fractions of Ae5 were tested for phytotoxicity. At the concentrations of 0.5 and 1% w/v of the partially purified culture filtrate fractions, all except the butanol 1 fraction, had no toxic effect on waterhyacinth leaf-segments. Butanol fractions at 10% (w/v) concentration were significantly more damaging than aqueous fractions (10% w/v) and the area of necrosis increased with time.     

Flower-visiting bees and the breakdown of the tristylous breeding system of Eichhornia azurea (Swartz) Kunth (Pontederiaceae)

Populations of Eichhornia azurea (Swartz) Kunth (Pontederiaceae) are often composed of three floral morphs that differ in style length and anther position. Hence, E. azurea is described as tristylous. For sexual reproduction the plant requires specialized pollinators able to transfer pollen from flowers with long-, mid-, or short-level anthers to flowers with long-, mid-, or short-styles, respectively. This is because within a floral morph, self-incompatibility is the rule. Populations of E. azurea were studied from six localities in south-eastern Brazil (São Paulo and Minas Gerais). The frequencies of floral morphs, floral morphology and flower-visiting bees were recorded in each population. Ten different bee species were observed visiting plants. Four of them were considered generalist bees and not appropriate for legitimate pollination of the three floral morphs. Another three species were considered opportunists and only three species were considered to be potential pollinators. At the coastal region (Juréia), the plant population lacked the short-styled morph and the mid-styled morph and exhibited a semi-homostylous condition, in which the low-level anthers were adjacent to the stigma. This morph showed high level of self-compatibility. The disappearance of the short-styled morph from this region is likely correlated with the absence of Ancyloscelis gigas Friese, a specialized long-tongued bee that is oligolectic on this plant in other localities.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 77 , 499–507.     

Deleterious effect of herbicides on waterhyacinth biocontrol agents Neochetina bruchi and Alternaria alternata

The integration of biological and herbicidal methods is advocated to manage waterhyacinth, but this can only be achieved when herbicides are nontoxic to the biocontrol agent. Therefore, laboratory experiments were conducted to determine the toxic effect of herbicides on the insect biocontrol agent, the waterhyacinth weevil, Neochetina bruchi Hustache, and phytopathogen, Alternaria alternata, with two commonly used herbicides, glyphosate and 2,4-dichlorophenoxy acetic acid at three recommended doses. The herbicides were sprayed on the waterhyacinth weevils and added to the nutrient media of A. alternata. 2,4-D caused higher weevil mortality (6.7, 13.3 and 15.6%) as compared to glyphosate (3.3, 5.6 and 11.1%), at three doses over 72 h. There was also a decrease in feeding in the herbicide treated leaves. When the weevils were allowed to move freely between the herbicide treated and untreated plants, higher orientation of the weevils was found on the untreated waterhyacinth than on the treated ones. Neither of the two herbicides actually killed the fungus but both inhibited its growth. Glyphosate though, delayed mycelial growth yet stimulated sporulation while 2,4-D inhibited both growth and sporulation. Glyphosate at low concentration did not affect the virulence of A. alternata, while fungi grown on 2,4-D amended plates lost their virulence.     

Plectosporium tabacinum Root Rot Disease of White Lupine (Lupinus termis Forsk.) and its Biological Control by Streptomyces Species

Plectosporium tabacinum root rot disease of white lupine is reported for the first time in Cairo Governorate provinces. Symptoms of the disease appeared on young seedlings as stunting, yellowing and wilting of the foliage. Rotted roots were first became light brown and the lesions progressively extended over the whole root system and became dark brown. Plants died when the whole root system had become infected. For successful biological control of the disease 70 rhizosphere actinomycetes were isolated by the use of normal standard isolation techniques. Three of these isolates were found to be strongly antagonistic against P. tabacinum in vitro . They also significantly reduced the incidence of white lupine root rot disease in soil infested with P. tabacinum in greenhouse trials. The three actinomycete isolates were identified as Streptomyces cyanoviridis , Streptomyces murinus and Streptomyces griseoplanus .