Plant Health Update, July-August 2025 Summary

Although temperatures were mild in early July, hot and dry weather was present by the middle of the month and throughout August. These conditions can contribute to plant stress, and many home gardeners started to notice damage from spider mites (Family Tetranychidae), which are attracted to stressed plants.  The mites feed on the contents of individual plant cells, causing them to die. The result is a “sanded” or “stippled” appearance of the foliage (Figure 1). Many types of plants are affected by spider mites, and home gardeners should inspect leaves for mites and webbing. Webbing can be severe in some cases; however, it is not uncommon for two plants of the same type to have different susceptibility to spider mites (Figure 2). This may be due to other pests, disease, or environmental stress factors (amount of light, nutrition, proximity to other hosts, etc.). The two-spotted spider mite (Tetranychus urticae) is the most common mite observed in Oklahoma.

 

Figure 1. Tomato leaf with stippling and webbing from spider mites.

 

Figure 2. The marigold plant on the right (orange circle) is infested with spider mites. Note extensive webbing and stippling of the leaves. The marigold plant on the left (black circle) is unaffected.

 

Spider mite populations are best managed using integrated pest management practices. It is important to maintain balanced fertility of plants, as too much or too little nitrogen can allow spider mite populations to increase. To minimize plant stress, provide supplemental irrigation when needed. Avoid growing susceptible plants in locations with reflected heat (near walls or pavement). Additionally, crowded plantings are more prone to mite outbreaks than less dense plantings.  Keeping plants well-spaced, clean, and free from debris or dying plant material can help prevent spider mite infestations. Heavily stippled or dying leaves can be trimmed and discarded in the trash (avoid leaving pruned plant tissues near healthy or unaffected plants). Some spider mite populations can also be reduced temporarily by rain or forcefully washing the leaves with water. For heavily infested plant tissues, however, discarding the entire plant in the trash may be necessary.  

 

Spider mites are poorly controlled with most insecticides. Some insecticides (i.e. Pyrethroids) can result in an increased population of spider mites, as predatory arthropods or natural enemies (lady beetles, green lacewing larvae, predatory mites, etc.) are killed by these products. If chemical control is employed, contact insecticides (miticides) directly impacting mite life stages should be used. During application, it is important to uniformly coat lower leaf surfaces, since many mites and mite eggs can be found on the underside of the leaves. Active ingredients such as horticultural oil and insecticidal soap are often used; however, these products can damage plant foliage by burning leaves when applied in high temperatures (phytotoxicity). 

 

In Oklahoma, spider mites are especially problematic on crops such as tomatoes.  In late July-August, cutting back the foliage to remove stippled and brown leaves is the best management strategy. After pruning, fertilizing the plants will stimulate new growth. When temperatures cool in August, new blooms may set. These steps ensure many home garden tomatoes recover in time to produce a fall crop, when cooler temperatures and rainfall are less favorable to spider mites.

 

The PDIDL received one particularly interesting sample in July-August. Carla Smith, Horticulture Extension Educator in Pottawatomie County, submitted images (Figures 3-4) showing the lower trunk of a crape myrtle (Lagerstroemia indica) shrub with unusual fungal growth. It was indicated that the plant appeared to be healthy when the images were taken.

 

Figure 3. View one of crape myrtle stems with unusual fungal growth at the base of the plant. Image by Carla Smith, Pottawatomie County Extension.

 

Figure 4. View two of crape myrtle stems with unusual fungal growth at the base of the plant. Image by Carla Smith, Pottawatomie County Extension.

 

Lab diagnosticians were unable to identify the fungus from the images alone, so a physical sample was submitted to the lab.  Using molecular diagnostic methods (PCR and DNA sequencing), the fungus was identified as a Sebacina species. This fungus grows on organic material in or on the soil and is sometimes associated with plant stems.  It is not harmful to the plant, and is sometimes beneficial, as it is reported as a mycorrhizae fungus on some hosts. There was no need to be concerned about the fungus; its growth was a result of high soil moisture and humid conditions.

 

Included in this report is a summary of plant health problems from Oklahoma specialty crop growers (horticultural crops). Table 1 shows the submissions for the month of July, and Table 2 shows the submissions for the month of August.  When possible, the specific type of plant is indicated in the host column of the table.

 

 

 

 

Look for additional plant health updates from the PDIDL in the next few weeks as the lab rounds out 2025.  If you have suggestions for future topics, please contact jen.olson@okstate.edu or call the PDIDL at (405) 744-9961.