The Morton Arboretum
Hawthorn and quince rusts caused by Gymnosporangium globosum and G. clavipes respectively, were evaluated on 37 Crataegus species and cultivars (133 trees) at The Morton Arboretum between 2001 and 2003. Trees were rated by two people each year during mid summer for: a) hawthorn leaf rust severity using a disease severity rating (DSR) scale of 0 (no symptoms) to 4 (75-100% of foliage with leaf spots), and b) the presence or absence of quince rust on fruit, stems or thorns. Statistical comparisons of the average DSR values indicate hawthorn rust was significantly worse overall in 2003 (DSR = 2.5) compared to 2002 and 2001 (1.2 and 0.9, respectively). Crataegus altaica, C. crus-galli, C. crus-galli var. inermis, C. marshallii, C. phaenopyrum and C. sanguinea were highly resistant (DSR = 1.0) to hawthorn rust in 2003 when disease pressure was greatest. Additional taxa exhibited moderate resistance in 2003 and a high level of resistance in 2001 and 2002. Despite being listed as tolerant taxa, green hawthorn (C. viridis) and Lavalle hawthorn (Crataegus x lavallei) were susceptible (DSR = 3.0 and 4.0, respectively, in 2003. Quince rust was similar to hawthorn rust in being more prevalent in 2003 than other years. In 2001, no quince rust-was found on several taxa: C. crus-galli var. inermis, C. pentagyna, C. pseudoheterophylla ssp. pseudoheterophylla, and C. viridis,. However, all taxa developed quince rust in subsequent years. These results suggest that there is ample room to further explore resistance to leaf rust but are less encouraging for quince rust.