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In a broad sense , the geometrical optics model is best suited for very rough surface , the physical optics model is suitable for surfaces with intermediate scales of roughness , and the small perturbation model is suitable for smooth surfaces . The third requirement is multitemporal coverage . Control of this disease is best accomplished by preventing the entrance of phytoplasmas into the garden . In western Minnesota , where the native pH of the soil is greater than 7.0 , amending the soil to a suitable range is not possible . For students taking the module in semester two , these lectures will be delivered on-line . Plants do not store at all well after this dipping regime , so it is best to treat plants about the time they would be breaking dormancy . Sanitation - - or removal of any spent flower blooms , infected buds , leaves and stems - - is best done during a dry , calm time of the day . This material reacts much faster than elemental sulfur ( less than 1 month ) ; however , the cost is greater .