There are funguses for most any plant but I suspect your plant's problems are in the roots. Dry soil is certainly a problem and if the plants are newly planted (less than 6 months old) the roots are still fairly confined and thus the root ball dries out quickly in this summer heat. Soggy soil is also a possible cause. If the soil is kept too wet and especially if the soil drains poorly the planting hole can tend to hold water, thus depriving the roots of oxygen leading to their death. Root rots may often follow such conditions adding insult to injury.
Dig down a few inches around the root ball and feel the soil to get an idea how often to water. Allow it to dry just a little between waterings.
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