Desert mistletoe ( Phoradendron californicum) infestation correlates with blue palo verde ( Cercidium floridum) mortality during a severe drought in the Mojave desert

Effects of plant parasitism on plant hosts are dependant upon cumulative biotic and abiotic environmental conditions. Blue palo verde ( Cercidium floridum, Leguminoceae) is parasitized by desert mistletoe ( Phoradendron californicum, Viscaceae) across its range. Mistletoe infestations are positively correlated with host tree height, and not with host mortality in the Sonoran Desert, but this system has not been monitored previously in the Mojave where water availability is less predictable. We mapped the distribution of 1212 live palo verdes in June of 2001 and 77 dead trees in January of 2002 and quantified parasite load on an 80 ha site in California. Blue palo verde mortality is highly positively correlated with mistletoe infestation, host height, and interaction of host size and plant parasite load in this study. 96.7% of trees without mistletoe were alive at the conclusion of the study, compared to only 38.6% of trees with mistletoe infestations. An SPI drought index of the area shows that droughts occur in an episodic manner, potentially causing periodic large-scale palo verde mortality during periods of severe drought. Results highlight the necessity to monitor plant host–parasite interactions across their range as interactions are apt to change with varying environmental conditions.