470 ROBERT C. HARE 



PHYTOALEXINS 



Phytoalexins are antibiotic substances produced by the host in 

 response to infection (Cruickshank, 1963a, b) . Resistance is dependent 

 upon the host's ability to form phytoalexins above the tolerance level 

 of the fungus. Other workers have not reported phytoalexin production 

 in pine in response to rust infection. 



To see if phytoalexins might te involved in shortleaf pine resistance 

 to fusiform rust, Hare (1970) planted three 6-inch pots of soil thickly 

 with seeds of shortleaf pine and three with slash, to form "lawns" of 

 seedlings. After seedcoats were shed, the seedlings in the first pot of 

 each species were misted with distilled water, tnose in the second pot 

 were misted with a water suspension of basidiospores , and those in the 

 third were misted with water, then covered with telia-bearing oak leaves. 

 All were then incubated over water in closed jars at 20°C for 72 hours. 

 The water droplets were blotted off the seedlings with filter paper 

 and the paper was eluted with water and organic solvents. The combined \ 

 washings were filtered and concentrated at 40°C. The aqueous residue 

 was tested at concentrations of 0.1, 1.0, and 10.0 percent in 0.5 percent 

 water agar against basidiospore germination and germ-tube growth. The 

 agar was buffered to induce direct germination. Control plates contained 

 agar made up with water or the filtrate from a spore suspension not 

 placed on foliage. Each test was replicated three times. 



Pine diffusates from seedlings sprayed with a basidiospore suspension 

 were not as active as those from seedlings inoculated directly by spores 

 as they were abjected from telia. The control filtrate from a spore sus- 

 pension not placed on foliage had little effect, even at a 10 percent con- 

 centration. Growth of germ tubes was usually promoted by the diffusates, 

 up to 1,100 percent by some treatments, but spore or species effects on 

 germ tube growth were not statistically significant at the 0.05 level. 

 Basidiospore germination on the water-agar control averaged 90.9 percent. 

 Table 1 shows the effect of discharged spores on the activity of diffusates 

 from the two species, when incorporated into 0.5% water agar at three levels' 



At 10 percent concentration, water diffusate from shortleaf was 

 three times as inhibitory to spore germination as that from slash, but 

 spore diffusate from both species inhibited germination of over 99 percent 

 of the spores. The results from the highest concentration indicate a 

 preformed fungitoxin that is quantitatively correlated with resistance, 

 and production of a phytoalexin by both resistant and susceptible tissue. 



Table 1. Effect of diffusates in agar on germination of 

 basidiospores. Water agar control germination 90.9% 



Concentration Spore diffusate" Water diffusate ~ Spore/water ratio" 

 of diffusate Slash Shortleaf Slash Shortleaf Slash Shortleaf 



0.1 



98.5 



88.0 



84.5 



74.5 



116.1 



118.1 



1.0 



93.4 



68.6 



72.9 



92.1 



128.1 



74.5 



10.0 



0.8 



0.2 



19.7 



6.2 



4.1 



3.2 



> 



