What Makes a Plant Have Brown Spots?

Fungi

• Especially in turf grasses, fungi often cause leaf spot. The three main genera are Bipolaris, Drechslera and Exserohilum, formerly all grouped under the name Helminthosporium. Spots begin as small brown patches and then progress to larger reddish-purple ovals with tan centers. Normally in turf grasses and other plants, these fungi cause minimal damage, but when plants are repeatedly mowed or clipped and then treated with high amounts of nitrogen fertilizer, the spots often get worse.

Bacteria

• More than170 bacteria can cause diseases in plant leaf tissues, many of which manifest as brown spots. These may be small and dense, or they may be larger, appearing to be rotted or water-soaked regions or elongated yellow-brown patches. Because bacteria are normally present on leaves, they will only manifest as unsightly spots when conditions are favorable to their growth, including crowding, poor air circulation and high moisture levels. There is such a wide variety of causal agents that it’s best to treat the plant holistically rather than try to pinpoint the virus.

Moisture Problems

• Overwatering turf grasses and foliage plants can lead to bacteria and fungal proliferation where, without excess moisture plants, would be healthy. Watering constantly as opposed to on a schedule coats leaves in water, which encourages the growth of harmful pathogens. On the other hand, watering too infrequently depletes plants of resources and makes them more susceptible to disease as well. Always consider the plant’s needs before you water, and allow soil to dry out between soakings.

Preventing Leaf Spot

• Proper plant care will help correct as well as prevent leaf spot. In addition to a regular watering schedule that takes each plant’s specific needs into account, you can decrease the incidence of leaf spot with other methods. Provide plants with good air circulation by pruning them correctly to allow light and breezes into the inner branches. Resist the temptation to grow turf grasses or other sun-loving plants in the shade, where they are less healthy. Don’t mist plants or otherwise provide extra moisture, and try not to let water splash from the leaves of one plant onto another, which could spread disease.