472 ROBERT C. HARE 



Chiba (1966) reported a negative correlation between the sucrose: 

 reducing sugar ratio and resistance of poplar to Melampsova laviei- 

 populina Kleb. Supplying 5 percent sucrose seemed to increase suscep- 

 tibility. In addition to being a C source for fungi, sugars may complex 

 with phenols to form more stable glycosides; high N may increase com- 

 plexing of proteins with phenols. In both cases, resistance would be 

 lowered by inhibiting quinone formation. High N content has been fre- 

 quently associated with decreased resistance, while high K increases it. 

 Trolldenier (1969) has reviewed the literature on cereal rust resistance 

 and mineral nutrition. The N:K ratio is most important. Increasing K 

 along with N frequently increases both yield and rust resistance. Excess 

 N may increase the pathogen's supply of substrate in the form of amino 

 acids and soluble carbohydrates. K deficiency tends to have the same 

 effect. It increases the activity of hydrolases and restricts phos- 

 phorylation, causing amino acids and soluble carbohydrates to accumulate. 

 High K promotes synthesis of insoluble carbohydrates and proteins, 

 reducing the substrate pool for pathogens and increasing mechanical 

 resistance to penetration. 



In a field study with fusiform rust, N fertilization and cultivation 

 quadrupled the number of cankers (Boggess and Stahelin, 1948), but the 

 effect was probably indirect through stimulated production of rust- 

 susceptible tissue when production of basidiospores was highest. 

 Hutchinson (1935) and Hanover (1963) reported higher K levels in rust- 

 resistant pine tissues. Fertilization with N decreased resistance of a 

 poplar clone resistant to Melampsova rust and increased susceptibility of 

 a susceptible clone (Donaubauer, 1966). Since NPK had no effect, N 

 fertilization may have reduced K content of the leaves. In wheat, rust 

 susceptibility seems to be associated with a high amino-N content or 

 with a low C:N ratio (Barrett and McLaughlin, 1954). In general, 

 NH4NO3 promotes susceptibility much more than KNO3 , and K fertilization 

 alone promotes resistance (Trolldenier, 1969) . 



HOST EFFECT ON PATHOGEN 



EFFECT OF DIFFUSATES AND EXTRACTS ON SPORE GERMINATION 



Fusiform rust basidiospores placed on distilled water agar with a pH 

 of about 6 germinate indirectly, forming daughter spores (secondary, 

 tertiary, etc.) instead of germ tubes (Kais, 1963). However on water at 

 pH 6 they germinate directly, forming long germ tubes. This "agar 

 effect" can be reversed by buffering to pH 5 or below (Hare, 1970) . 

 Daughter spores also form on non-host tissue (Miller and Roncadori, 

 1966). The spores may be abjected as far as 0.3 mm. A similar response 

 has been reported with C. vibieola basidiospores (Bega, 1960) which form 

 germ tubes at pH 3 to 5, mostly daughter spores at pH 6 and above. 

 Germination is direct on white pine needles and indirect on nonhost pine 

 needles. Daughter spores may have survival value if they fall from 

 resistant to susceptible tissue and then germinate directly. 



Tops of 1-month-old susceptible slash pine or resistant shortleaf 

 seedlings or both were embedded in water agar which was then seeded with 

 basidiospores (Hare, 1970). Only those spores adjacent to the slash 

 seedlings germinated directly, whether or not shortleaf was also 

 adjacent. These results indicate a germination promoter diffusing from 

 susceptible tissue, with no effective inhibition from resistant tissue. 

 When an indicator solution was added to the agar, a yellow zone (pH 4 



