Unfortunately there is nothing you can do except to wait to see if the plant can regrow lost branches over time. In that case, trim off all dead or wilted branches since individually they will not recover. Otherwise, the shrub will need replacing.

As to the original cause, it's hard to say since symptoms like wilting and branch death can have several different causes. Our rhododendron diagnostic page presents some possibilities, but it's not an exhaustive list. Usually, such damage is tied to plant stress, such as from drought or soil oversaturation, but occasionally a stressed plant will fall victim to a stem-boring insect or opportunistic infection that kills branches or roots faster than the stress alone would. Soil conditions are suspect because rhododendron and their azalea cousins can be quite sensitive to soil moisture extremes, though soggy conditions tend to cause more rapid dieback than too dry. Both will cause wilting though, with leaf browning and branch death if those conditions persist too long. Neither a fungicide nor an insecticide would be of use once such symptoms appear.

When monitoring a shrub for watering needs during dry weather, feel the soil around six inches deep (maybe four or five inches for rhododendron given their somewhat shallower roots). When it's becoming relatively dry to the touch at that depth, water thoroughly, especially if the plant is sharing the root zone of a nearby mature tree. As a very rough guideline, this might amount to one soaking per week during very hot, dry weather in summer, but much will depend on soil type and other site conditions, which is why checking the soil before watering is important. If you mulch around the plant, make sure the mulch isn't covering any of the plant's trunk/stems.

If a roof downspout outlet empties nearby, see if it can be directed elsewhere so that surplus water doesn't flood the root zone, or if you replant, consider using a shrub species more tolerant of temporary soil sogginess. Our more erratic and heavier rainfalls exacerbated by climate change might make intolerable root zone flooding a more common occurrence for plants sensitive to such conditions. This might not be too big of a problem by itself, but combined with the compacted condition most residential soils have due to construction, it could lead to roots being starved of oxygen long enough to die back or become infected with root rot.

Miri

Hello Floyd,

This looks like Alternaria Leaf Spot, a common fungal infection on Zinnia. You can't do anything this time of year to stop the spread of infection or manifestation of symptoms, and while a fungicide used preventatively prior to symptom appearance next year might work decently well, we generally discourage fungicide use when it's not needed. Fungicides can potentially have negative impacts on other organisms and don't always work well, even if applied as directed. They will also need frequent re-application, which might be more costly than just replacing the plants if the infection is too damaging or unsightly. For cut flower use, growers dealing with these types of infections just strip the leaves off of zinnia stems used in bouquets and use other foliage as filler if needed.

Like many fungal and bacterial leaf infections, wet leaf surfaces increase the plant's vulnerability to infection. When you water the zinnias if they are in need of irrigation, do so by only wetting the soil and not the leaves, or if unavoidable, water early enough in the day so the foliage can be dry by nightfall. Granted, you can't do much about rain or dew wetting the leaves, which is why some years an infection might outbreak like this while in other years go largely unnoticed. Remove any dead fallen leaves when you are done with the plants this season (such as once our first frosts kill them) and dispose of them to avoid the debris harboring spores to spur another infection next year.

Miri

Thank you, and you're welcome. We have both Master Gardener volunteers and horticulturist staff answering Ask Extension questions; I happen to be one of the latter. Education is our goal! Happy to be of service.