Applications of Potassium Silicate Decrease Black Spot Infection in Rosa hybrida `Meipelta' (Fuschia Meidiland™)

Roses in nursery and landscape settings are frequently damaged by black spot, whose caus al agent is the fungus Diplocarpon rosae F.A. Wolf. Potassium silicate was as- sessed as a media-applied treatment for decreasing the severity and incidence of black spot infection. Roses were treated with 0, 50, 100, or 150 mg·L-1 silicon as potassium silicate in cor po rat ed into ir ri ga tion water on either a weekly or daily schedule. Five weeks after treatments were initiated, plants were in oc u lat ed with D. rosae. Roses began to show vi- sual symptoms of infection s 4 days later. Roses that had 150 mg·L-1 silicon applied on a daily schedule had significantly more silicon present in their leaves than other treatments as mea sured by scanning electron microscopy and energy-dis per sive x-ray analysis. In ad di tion, roses that had 100 and 150 mg·L-1 silicon ap plied on a daily schedule had few er black spot lesions per leaf and fewer in fect ed leaves than any of the other treat ments by the end of the ex per i ment 7 weeks later. Although roses treat ed with high er levels of sili- con on a daily basis fared better than roses in the other treatments, all of the roses were heavi ly in fect ed with D. rosae by the end of the study. The results reported here indicate that us ing potassium silicate in ir ri ga tion water may be a useful component of a dis ease man age ment sys tem. been shown to increase with these additions in cer tain crops, including rice, cucumbers, bar ley, and sugarcane (Ayres, 1966; Dyke, 1979; Epstein, 1999; Miyake and Takahashi, 1983). Supplemental applications of silicon are ben e ficial to crops not only because silicon is considered a necessary element for the healthy growth of many plants (Cheng, 1982; Epstein, 1999), but also because silicon additions af fect soil properties such that there is an in creased availability of media phosphorus (Mengel and Kirkby, 1987). Foliar applications of silicon com pounds on grapes, cucumbers, zucchini, musk mel on, and rose have revealed that this ap pli ca tion method may also be useful for improving plant defenses (Bowen et al., 1992; Gillman and Zle- sak, 2000; Menzies et al., 1992). It has been speculated that foliar applications of sol u ble silicon may serve to increase plant resistance to disease by absorption of silicon through the leaf (Menzies et al., 1992), or by the creation of a protective silicon barrier on the surface of the leaf (Bowen et al., 1992). Black spot of rose, caused by the fungus Diplocarpon rosae, is the most trou ble some disease of roses worldwide (Horst, 1983). Outdoor production situations, common in nurseries, provide free water for spore dis- pers al and germination. Susceptible cultivars can quickly defoliate, making them unsalable. Previous research has shown that foliar ap- pli ca tion of sodium silicate to rose cuttings de creas es leaf drop and black spot infection as well as increasing rooting of cuttings (Gill- man and Zlesak, 2000). The effects of soluble silicon on resistance to black spot when ap plied to the media in which the roses are growing,