450 INSECTS AFFECTING VEGETATION 



tans, upon the potato crop. It is understood that plenty of moisture 

 is the usual accompaniment of a cool season; from the combined 

 effect of this supply of moisture and cool weather we have outbreaks 

 of the potato disease. 



CAUSES OF PLANT DISEASE. 



Non-Parasitic Troubles. 

 Atmospheric : 



Excessive Cold; 



Excessive Heat; 



Excessive Dry ness; 



Excessive Moisture ; 



Wind; 



Hail; 



Artificial Influences. 

 Soil: 



Deficiency of Necessary Substances ; 



Excess of Certain Substances (Alkali, Salt, Acid) ; 



Excessive Dryness; 



Excessive Moisture; 



Faulty Physical Structure (Hard-pan, Heavy or Coarse 

 subsoil) ; 



Artificial Influences. 

 Parasitic Troubles. 



Higher Animals (Gophers, Moles, Squirrels) ; 



Insects ; 



Worms; 



Mpllusks (Slugs, Snails) ; 



Higher Plants (Dodder, Mistletoe) ; 



Fungi; 



Bacteria ; 



Slime Moulds. 



The above table indicates most of the influences which affect 

 plants injuriously in California, so far as they can be definitely tabu- 

 lated. It should be further mentioned that, in many cases, various 

 combinations of the above have disastrous results which could not be 

 attributed to any influence acting alone. 



DIRECTIONS FOR EXAMINATION OF DISEASED PLANTS. 



Before proceeding to a description of the principal troubles of our 

 various crops it may be useful to give the following general directions 

 for the examination of unhealthy plants: 



1. Note whether any particular portion of the orchard or field 

 shows the trouble worse or less and, if so, seek to find wherein condi- 

 tions are different there from those in the remainder of the planting. 



2. Note whether any particular kind or variety of the crop is 

 more or less affected than others, also whether any particular indi- 

 viduals show marked resistance, immunity or freedom from the 

 trouble. Ascertain the commercial qualities and desirability of any 

 seemingly resistant or immune variety or individuals. 



3. Determine as accurately as possible the part of the plant which 

 is actually affected. In many cases, for instance, the leaves or top 



