Annual zinnias (Zinnia spp.) are brightly coloured flowers that need little care and thrive in direct sunshine and warm temperatures. Despite the fact that zinnias have a high level of resistance to illness, they are sometimes affected by diseases that result in brown blotches. The brown patches on the leaves are often brought on by one of three different kinds of leaf spot: bacterial leaf spot, Cercospora leaf spot, or Alternaria leaf spot. When left unchecked, all three will lessen the plant’s aesthetic appeal and might ultimately lead to the plant’s demise.
According to the Missouri Botanical Garden, zinnias may be grown successfully as annuals in USDA hardiness zones 2 to 11. These plants do best when grown in full sun and in soil that is both well-drained and wet.
Alternaria Leaf Spot
Zinnias, along with numerous other types of plants, including tomatoes, lettuce, sunflowers, and asters, are susceptible to the fungal disease known as alternaria leaf spot. According to research conducted at Clemson University, the illness may be identified by the presence of tiny, reddish-brown spots that have tan or grayish-white cores. The tips of the leaves are where you will find the spots, which start off little but quickly get bigger and darker in tone. After some time, the centres of the spots will fade away, leaving behind irregularly shaped holes. If the condition is not addressed, the leaves may wither and become brittle and dry.
This illness may also have an effect on the blooms and the stems of the blossoms, which can deteriorate and finally die after developing lesions. This disease may be passed on from infected plant to plant via the seeds.
Bacterial Leaf Spot
Bacterial leaf spot is a bacterial illness that may be identified by the presence of tiny spots that are water-soaked and transparent and can be seen on the flowers, leaves, and stems of the plant. The specks are surrounded by distinct halos that have a yellowish cast. When exposed to high levels of humidity, the spots become bigger and their centres take on a rusty brown colour. When the condition is severe, the spots combine to form huge lesions that spread throughout the leaf’s surface. Cankers appear on the stems, and the blooms take on an abnormal appearance. In due time, the plant will wither, get mushy, and eventually rot away.
Cercospora Leaf Spot
Both the Cercospora leaf spot and the Alternaria leaf spot are related diseases, and it is possible for a plant to have both illnesses at the same time. Cercospora leaf spot is quite similar to Alternaria leaf spot, however the spots caused by Cercospora are bigger, more rusty brown or purple in colour, and it mostly affects the older, lower leaves. Alternaria leaf spot is caused by the fungus Alternaria. As the growth season continues, the older leaves on the plant fall off, and the only leaves that remain on the plant are the newer, top ones.
Controlling Zinnia Diseases
Zinnias are susceptible to a number of different illnesses, the most of which are brought on by either incorrect care or an excess of moisture. Zinnias need to be grown on soil that is well-drained, in a lot of open air, and in direct sunshine. It is recommended to provide 1 1/2 to 2 feet of space between bigger cultivars, and just 1 foot between smaller cultivars. It is important to give plants enough of water when the weather is warm, but you should let the soil dry out in the time between waterings since wet soil encourages the growth of bacteria and fungi. Apply water to the soil at the plant’s base to prevent dew from forming on the foliage and to ensure that the space surrounding the plant is clear of dead leaves and any other debris. Remove and dispose of the diseased components of the plant as soon as possible.
During times of warm and humid weather, commercial fungicides are often used to treat severe instances of the leaf spots caused by Alternaria and Cercospora. At the first indication of illness, apply fungicides to the affected area. Spraying plants once a week in the late evening while they are in bloom is something that is recommended by Clemson University. Spraying during wet seasons is recommended for treating Cercospora Leaf Spot.
There is currently no effective commercial spray available to combat bacterial leaf spot.