September Horticulture Questions from Chouteau County Clientele
• I didn’t get my pumpkins covered when we had our first frost. Should I throw them away? According to NDSU Extension, the first light freeze usually will take out the canopy foliage and leave the fruit pretty much unharmed. Ideally, the crop should be harvested as soon as possible. If a prompt harvest isn’t possible, then hope for a week to 10 days of balmy weather with no further freezes. Harvest the pumpkins and squash at your earliest convenience. Frost blankets are highly recommended for gardeners because they are not expensive and are reusable for several years.
• What species of cottonwood in Chouteau County is infested with rust? Noelle Orloff (Montana State University Extension Plant Identification Diagnostician) identified the sample as lanceleaf cottonwood (Populus x acuminata), which is thought to be a hybrid between eastern cottonwood and narrowleaf cottonwood (P. angustifolia).
• What kind of rust are we dealing with? According to Colorado State University Extension, The rust is a fungus called Melampsora medusa. Though common, M. medusa rarely causes serious damage to trees since the disease develops late in the summer/fall and rarely causes early defoliation. The disease damages leaves after most photosynthetic needs for the tree are completed. The most prominent symptoms are pustules on the underside of the leaves, containing masses of bright-orange-yellow spores. Trees highly susceptible to M. medusa may experience premature leaf drop during late summer. M. medusa can be fatal to young trees. The life cycle of this fungus requires two different tree hosts. During wet spring weather, spores are released from the fungus, which has overwintered on fallen cottonwood or aspen leaves. Spores infect evergreen needles, such as Douglas-fir, pine, fir or spruce. Evergreen species are minimally affected by M. medusa. After two to three weeks, spores are produced on evergreen hosts, and are blown to aspen or cottonwood leaves. Once the rust is established on aspen or cottonwood hosts, it can multiply rapidly under favorable wet conditions throughout the summer. Several years of heavy infections can cause some growth losses, especially on younger trees. Fallen infected leaves shelter the fungus until the next year’s disease cycle.
• What do you recommend for management of rust on lanceleaf cottonwood? The best method of control is to plant resistant varieties. If you have huge trees like we do in Fort Benton, the only management recommendation is to dispose of leaf litter in order to reduce inoculum levels. Younger trees can be sprayed with a fungicide labeled for M. medusa.
