Fungus Flora of Some California Soils in Relation to Slow Decline of Citrus Trees

I N many of the older citrus groves of California it is becoming increasingly difficult to maintain normal production. After citrus trees have been grown on some soils for a considerable number of years, there comes a time when yields begin to diminish, the trees become unthrifty, abnormal dieback occurs, and new growth is below normal, despite the fact that standard fertilizer and pest-control practices are followed and management in general is good. When the old trees are replaced with young ones, growth of the replants is slow compared with that of similar young trees planted in noncitrus soil, or soil which has never before been used for citrus. This poor growth of citrus in many old orchards is commonly referred to as "slow decline of citrus". It is highly probable that slow decline of citrus trees is the result of a combination of factors, such as nutritional deficiency, nutritional unbalance, deterioration of soil structure, accumulation of toxic substances in the soil (2, i<5), and an unfavorable microbial population in the soil. In some instances above-ground conditions may be involved; however, greenhouse fertilization and soil-fumigation experiments carried out by the author, and field trials (5, 17) conducted by several large commercial citrus ranches, point strongly to soil conditions and suggest that a biological factor is at least partly responsible. The principle microbiological agents in ,the soil which might affect citrus growth are (a) nematodes (3> 5. 9> 2 I )> chiefly the citrus-root nematode, Tylenchulus semipenetrans Cobb; (b) pathogenic microorganisms (6, ii) ; and (c) saprophytic soil microorganisms. The possible relations of nematodes and of strictly pathogenic microorganisms to citrus decline are now under investigation in the Division of Plant Pathology at the Riverside Experiment Station.