Histopathology of Seed-Borne Infections - Applied Research Center ...

9Physiogenic orNonpathogenic SeedDisordersSeed disorders in which no recognizable pathogenic incitant is associated are knownas physiogenic or nonpathogenic diseases. Fungi, bacteria, viruses, mycoplasma,nematodes, and spermatophytic parasites are common pathogenic causes of plantdiseases. Nonpathogenic abnormalities are caused by disorders in the physiology ofplants due to unfavorable environments including soil conditions.Physiogenic diseases may be induced by (1) unfavorable soil conditions thatmay be due to the deficiency of essential elements, macroelements (nitrogen, phosphorus,calcium, magnesium, potassium, and sulfur), or microelements (iron, manganese,boron, zinc, copper, and molybdenum), adverse water relations (draught,water logging, and impeded aeration), and adverse physiochemical conditions (alkalinity,acidity, and salinity); (2) climatic stresses, including temperature, light, andhumidity; and (3) atmospheric environmental pollutants (gases such as ethylene,ammonia, sulfur dioxide, nitrogen dioxide, hydrogen fluorides, and particulates).Wallace (1951) has given a good account of diseases caused by mineral deficienciesin plants. Scaife and Turner (1984) and Bennett (1993) have discussednutrition deficiencies and toxicity in crop plants. Levitt (1972) has provided informationon the responses of plants to environmental stresses. In recent years, considerableinformation has been generated on air pollution and injuries to plants.Some of the important publications are those of McMurtney (1953), Darley andMiddleton (1966), Jacobson and Hill (1970), Lacasse and Treshow (1976), Pell(1979), Lawrence and Weinstein (1981), and Evans (1984). Agrios (1988) hasincluded a chapter on environmental factors that cause plant diseases in his book,while Neergaard (1979) has given a good account of nonpathogenic/physiogenicseed disorders.Aerial parts of plants, particularly growth and features of stem and leaves, arecommonly affected by nonpathogenic factors. Some of them also affect the reproductivephase, causing failure of seed formation. Only a few cause notable disordersin seeds (Table 9.1). Seed disorders for which adequate histological information isavailable are described under separate headings.247