Many diseases infect roses. The most common and serious, from an aesthetic standpoint, are foliar diseases. These include blackspot, powdery mildew, leaf spots, and rust. Severe infections of any of these diseases reduce plant vigor. Gardeners who plant roses susceptible to them are ultimately forced to choose between a program of regular fungicide spraying or acceptance of roses as landscape liabilities from mid-summer through autumn.

'Adelaide Hoodless' produces brilliant red blooms which cover the plant in June (left). Unfortunately, the cultivar is very susceptible to blackspot. Without fungicidal protection, it can be completely defoliated by mid-summer (right).

Slight, moderate, and heavy infections of blackspot are illustrated, top to bottom, in this series of photos.

Blackspot is diagnosed when small, circular, black spots with feathery margins develop on upper leaflet surfaces. 10 Spots are 1/8 to 1/2 inch (2 to 12 mm) in diameter. The leaf tissue surrounding these spots turns yellow and this chlorosis spreads until the leaflet drops from the plant.

Less noticeable black spots can also occur on a plant‘s petioles, stipules, peduncles, fruit, and sepals. Flower petals may be distorted and red flecks may occur. Raised, purple-red blotches that later blacken and blister develop on the immature wood of first-year canes.

Expanding leaves between 6 and 14 days of age are most susceptible to blackspot infection. The optimal temperature for disease development is 75° F (24° C). Conidia, the infecting spores, must be continuously wet for at least seven hours for infection to occur.

The fungus overwinters in infected canes and in fallen leaves. Protective fungicidal sprays and planting of resistant cultivars are the best means of blackspot control.

Slight, moderate, and heavy infections of powdery mildew are illustrated, top to bottom, in this series of photos (right).

Powdery mildew is another widely distributed and serious disease of roses. Young tissues are the most susceptible and the disease is typically diagnosed when white, powdery patches of fungal growth appear on young leaves. 11 These leaves will often fold inward or become twisted and distorted. New stem growth and flowers can also be attacked.

Optimal conditions for disease development are daytime temperatures between 62° and 77° F (18° to 25° C), with 97 to 99 percent relative humidity, followed by a night temperature of 60° F (16° C) and a relative humidity of 90 to 99 percent. Although high humidity encourages disease development, powdery mildew will not develop well when leaves and other plant parts are wet. Conidia, the infecting spores, will not germinate in water.

The fungus overwinters in rudimentary leaves in buds or in the inner bud scales. Fungicide sprays provide the best control.

Slight, moderate, and heavy infections of leaf spots are illustrated, top to bottom, in this series of photos (right).

There are several leaf spot diseases of roses, including those caused by Alternaria, Cercospora, Colletotrichum, and Sphaceloma rosarum.¹²

Spot anthracnose is caused by Sphaceloma rosarum. Young spots are red, occasionally brown or purple, and occur on upper leaf surfaces. Spots are circular and up to 1/4 inch (.5 cm) in diameter. The center of spots turn gray or white. The center of the spot often falls out, leaving a shot-hole appearance. Heavy infections of spot anthracnose can result in yellowing of infected leaflets and will cause defoliation.

Conidia are formed in spring and continue to form on rainy days through the growing season. Spores are spread by splashing water.

Slight, moderate, and heavy infections of rust are illustrated, top to bottom, in this series of photos.

Nine species of the rust fungus Phragmidium occur on roses. 13 Rust is typically diagnosed when reddish orange pustules, containing spore masses, appear on lower leaf surfaces. As the pustules develop, they appear on the upper leaf surface as chlorotic spots, giving this surface a mottled appearance. At the Minnesota andscape Arboretum, leaves growing at the base of plants are the most commonly infected.

Temperatures between 64° and 70° F (18° to 21° C) are optimal for rust development. Continuous moisture for two to four hours is necessary for infection to occur.

Rust is not as severe of a problem in Minnesota as it is in the western United States and other areas with wet, cool growing seasons.

The incidence of blackspot, powdery mildew, leaf spots, and rust was monitored at the Minnesota Landscape Arboretum during the growing seasons of 1990 and 1992. Both of these growing seasons were cool and humid, providing excellent conditions for disease development.¹⁴The results of these observations are in Table 8.

Roses in the Shrub Rose Garden at the Arboretum are not protected with pesticide sprays, which allowed differences in disease incidence among cultivars to be observed. Observations were made from early August through late September. Disease incidence was recorded as none, slight, moderate, or heavy for each cultivar or species. Levels recorded in Table 8 are the highest seen for the year.

Leaf spot diseases were not differentiated during the observations. Symptoms are similar among several of the leaf spot diseases and literature that aids in differentiating among these diseases is scarce. Most infections appeared to be spot anthracnose.

The degree of infection on a field-grown plant is a function of several factors. A disease-free plant may indicate that the cultivar is resistant to the races of pathogens present, while infected cultivars are susceptible. Pathogenicity among Diplocarpon rosae races has been found to be cultivar dependent.¹⁵

On the other hand, an uninfected plant may be an “escape,” a plant that was missed by infecting spores because of non-uniform pathogen distribution.

Disease development is also affected by environmental variables, such as temperature, humidity, and air circulation. Differing levels of infection may simply represent localized, microclimatic variations that are conducive or detrimental to disease development.

Controlled screening techniques are necessary to separate genetic resistance from these other factors that influence disease development.

Some trends can be seen across evaluation years and among and within classes of roses. The levels of infection for all diseases monitored were generally higher in 1992 than in 1990. The levels of blackspot and powdery mildew on the Species Roses were the exception to this trend, with infections much more severe in 1990 than in 1992.

The year 1992 was a particularly severe one for leaf spot occurrence. Leaf spot infections were more prevalent and more severe among the Old Garden Roses than among other classes.

Rust infections at the Minnesota Landscape Arboretum were the most prevalent and severe among Hybrid Rugosa cultivars.

The Albas and the Damasks were two of the most disease-free groups of Old Garden Roses at the Minnesota Landscape Arboretum. Leaf spots were the most common disease problem. Blackspot and powdery mildew incidence were not common or severe. Rust was not present in either year. ‘Königin von Dänemark’ and ‘Belle Amour’ were the most disease-free Albas in 1990 and 1992. ‘Césonie’, ‘Kazanlik’, ‘Rose de Rescht’, ‘St. Nicholas’, and ‘York and Lancaster’ were the healthiest Damasks.

Blackspot, powdery mildew, and leaf spots were observed on Centifolias, but rust infections were rare. The most disease-free representative of this class in 1990 and 1992 was ‘Centifolia Variegata’.

The most prevalent diseases of the Gallicas were powdery mildew and leaf spots. Blackspot and rust infections were minor. The healthiest cultivars in 1990 and 1992 were ‘Alain Blanchard’, ‘Belle Isis’, ‘Charles de Mills’, ‘Duchesse de Montebello’, ‘Narcisse de Salvandy’, ‘Superb Tuscan’, and ‘Tuscany’.

Although powdery mildew is often noted as the most common disease of Moss Roses and other Old Garden Roses, blackspot and leaf spots were much more common than powdery mildew on the Moss Roses during the two years of monitoring. Because water on leaves and other plant parts discourages the development of powdery mildew, the higherthan-normal precipitation of 1990 and 1992 may have affected powdery mildew incidence. 16 ‘Henri Martin’ and ‘William Lobb’ were completely disease-free in 1990 and 1992. Other healthy Moss Roses were ‘Capitaine John Ingram’, ‘Communis’, ‘Gracilis’, ‘Nuits de Young’, ‘Perpetual White Moss’, ‘ Salet’, ‘Violacée’, ‘Waldtraut Nielsen’, and ‘White Bath’.

When present, the levels of leaf spot, powdery mildew, and blackspot among the Floribundas, Hybrid Moyesiis, Hybrid Perpetuals, Hybrid Spinosissimas, and Kordesiis were higher in 1992 than 1990. Blackspot was the most common and severe disease. The Floribundas were the most disease-free of these classes, bothered only by slight levels of blackspot. The Kordesiis, with their glossy foliage, were also healthy, although moderate levels of blackspot were seen on some cultivars in 1992.

Like their parent species R. nitida, ‘Aylsham’ and ‘Metis’ are susceptible to powdery mildew. When present, disease levels on the two Hybrid Suffultas, ‘Assiniboine’ and ‘Cuthbert Grant’, were slight, although ‘Assiniboine’ is very susceptible to rust.

Hybrid Rugosa cultivars are very resistant to blackspot and powdery mildew in comparison to other rose cultivars. Leaf spots can occur but are not common. Rust was the most common disease problem of the group. ‘Charles Albanel’, ‘Frau Dagmar Hartopp’, and ‘Thérèse Bauer’ were disease-free in 1990 and 1992. ‘Agnes’, ‘Blanc Double de Coubert’, ‘Delicata’, ‘Grootendorst Supreme’, ‘Pink Grootendorst’, and ‘Sir Thomas Lipton’ were also very healthy with only light infections of leaf spots or rust.

There are no identifiable trends in disease resistance among the Shrub Roses, probably due to the heterogeneous genetic backgrounds of this group. Diseases do occur, particularily blackspot and leaf spots. Although the summers of 1990 and 1992 provided excellent conditions for disease development, most infections on the Shrub Roses were slight to moderate. The exceptions to this were ‘Adelaide Hoodless’ with its high levels of blackspot and leaf spots in 1992; the high levels of black-spot on ‘Morden Amorette’, ‘Morden Cardinette’, and ‘Morden Ruby’ in 1992; and the high level of blackspot on ‘Praire Youth’ in 1990. ‘Applejack’ and ‘Summer Wind’ were disease-free in 1990 and 1992. ‘Bonica’, ‘Carefree Beauty’, ‘Chamcook’, ‘Champlain’, ‘Golden Wings’, ‘Haidee’, ‘John Franklin’, ‘Lillian Gibson’, and ‘Prairie Wren’ were also very healthy.

Species Roses, like the Shrub Roses, usually have slight to moderate levels of disease incidence. Rosa arkansana and R. rugosa kamtchatica can be noted for their high susceptibility to powdery mildew, and severe infections of blackspot were seen on R. canina, R. foetida, R. foetida bicolor, R. glauca, and R. woodsii in either or both of the years 1990 and 1992. Rosa primula and R. spinosissima altaica were disease-free in 1990 and 1992. Other healthy species were R. amblyotis, R. hugonis, R. laxa, R. mollis, R. multiflora, R. pomifera, R. rugosa, R. rugosa alba-plena, and R. setigera.

R. pomifera is a Species Rose with small, medium pink flowers. It is grown for its large ornamental hips, which have high Vitamin C content. R. pomifera is disease tolerant at the Minnesota Landscape Arboretum.

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