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.