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-speci®c biocontrol agent for managing waterhyacinth (Eichhornia cras- sipes) in Egypt. An important diagnostic characteristic of this fungus is the production of crimson-red phyto- toxic 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 pro- duction of pigments was obtained when cultures were grown on potato dextrose agar (PDA) with 20% dex- trose, 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 nonchro- matic, 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 Para®lm 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 Para®lm wrap- pings, led to lower levels of the red pigment(s) and the nonpigmented UVACs, to reduced mycelial growth, and a suppression of sporulation. Di€erent culture- ®ltrate fractions of Ae5 were tested for phytotoxicity. At the concentrations of 0.5 and 1% w/v of the par- tially puri®ed culture ®ltrate fractions, all except the butanol 1 fraction, had no toxic e€ect on waterhya- cinth leaf-segments. Butanol fractions at 10% (w/v) concentration were signi®cantly more damaging than aqueous fractions (10% w/v) and the area of necrosis increased with time.