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J.O. Kuti, G.V. Latigo, and J.O. Bradford

Soil-borne pathogens such as Macrophomina phaseolina (the causative agent of charcoal rot) and Phymatotrichum omnivorum (the causative agent of cotton root rot) contribute to mortality of transplanted guayule (Parthenium argentatum, Gray) seedlings in southern Texas. In order to select guayule genotypes for resistance to these pathogens, it would be useful to develop reliable greenhouse inoculation procedures for screening guayule seedlings. Twelve-week-old guayule seedlings (`11591', a USDA standard breeding line) were inoculated using two inoculation methods (soil-drenching and root-dipping) in two soil media (field soil and commercial soil mix). Plants were rated for disease severity 2 to 5 months after inoculation and pathogens were re-isolated from diseased plants to establish Koch postulates. The soil drenching technique, using field soil, caused rapid development of disease symptoms that were consistent with re-isolation frequencies of pathogens from the diseased plant tissues.

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G.V. Latigo, J.R. Smart, and J.O. Kuti

Guayule (Parthenium argentatum Gray) is a promising alternative to rubber tree (Hevea brasiliensis Muell. Arg.) for production of natural rubber in semiarid regions. For guayule to be commercially viable, substantial improvement in rubber yield is needed. Field studies were conducted on a dryland site in south Texas to evaluate productivity of selected guayule genotypes from Arizona and California. After 34 months of growth, no significant differences (p= 0.05) were found among the genotypes for rubber yield. However, rubber yields for most of the genotypes increased more than 3-fold from that of last year (1992) yields. Genotype `N9-5' from Arizona had the highest yield (1,239 kg ha-1). Survivability of the genotypes has progressively decreased over the years and survival rates for this year (1993) ranged from 48-25%.

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G.V. Latigo, J Smart, J.O. Bradford, and J.O. Kuti

Guayule (Parthenium argentatum Gray) is a promising alternative to (Hevea brasiliensis Muell. Arg.) for rubber production in semiarid regions. Substantial improvement in yield is needed to establish guayule as a competitive source of natural rubber. A 4-year field study was conducted on a dryland site in southern Texas to evaluate productivity of selected guayule breeding lines from Arizona and California. Plants were harvested at the age of 22, 34, and 46 months and analyzed for dry weight, resin content, rubber content, resin yield, rubber yield, and percent mortality. While significant differences (P = 0.05) were found for dry weight, resin content, and rubber content within the harvest dates and among the guayule lines, no significant differences were found for rubber content between the harvest dates for each genotypes. Phytomass was highly correlated (r = 0.94) with rubber yield. Survivorship of all the guayule lines decreased progressively over the experimental period and mortality rates ranged from 38% to 67 %. Guayule lines `UC102' from California and `N6-5' and `P3-1' from Arizona were ranked highest for all traits measured.

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C.D. Grote-Flores, G.V. Latigo, J.O. Bradford, and J.O. Kuti

Guayule shrub (Parthenium argentatum Gray) is a source of natural rubber resin and latex. Because guayule does not produce natural antioxidants, considerable amounts of rubber and resin are lost after harvest. The effect of long (2–7 years) cold storage on postharvest stability of rubber and resin contents in selected dryland guayule breeding lines were compared. While most genotypes tested showed significant decline in rubber and/or resin content during the storage, few genotypes consistently maintained or increased the amounts of rubber or resin content during storage. The mechanisms of postharvest degradation or synthesis of rubber and resin in harvested guayule plant materials need to be studied further.