Lavenders are normally as tough as old boots, so IF you think you have a fungus etc under the soil at the roots, why don't you dig one of these affected plants up, take it to the shade and lay out a newspaper or rag, then try tease away the soil from around the root, soon as you see the roots, it will become evident what if anything is wrong at the root, you will either see very few roots ( since planting in spring the plants should have made good root) you will find either the soil is far too dry by it sifting away like sand, if there is still enough moisture it will need to be shaken away and should look like a darker colour or you feel it has had a drink.
go back to the hole where you removed the plant and examine the area, does it look fertile, (lavender's don't like a rich soil) but it should look like a plant would want to be there.
Are there any insects around /in the hole, has any watering reached DOWN to where the roots are.
These are all guides for you to search for any root damage.
Next look at the top growth of the plant, use finger to snap off a bit of the foliage, does it just break away or did it bend a bit before you managed to break it, IF the leaf snapped, the plant is dying of thirst, especially in the heat you guy's have had this summer.
Does the plant still smell Lavender, if yes, this tells you the lavender oil held within the plant is still flowing and the plant is OK. Look at the flowers, are they going brown early, are they dull in colour and looking sad, this points to under watering. it is hard to find the right care for plants in your heat this year but, I would be inclined to poke my fingers into the soil at the root area, if it feels dry, then give a good drink (remember in your heat most of the water will evaporate before the roots take it up) if it's wet, then you don't water, ALSO, it is important in your heat and lavenders/ any plants with hairy leaves, don't water the foliage, make sure you just aim the water jets around the soil.
The advice you got is the right advice regarding not to give too much water to Lavenders, they are plants that thrive in the Mediterranean regions where they like dryer conditions, but there is dry and arid desert conditions, so in the heat you have I would increase the watering, try do it at evening where the sun is a bit cooler so the water don't evaporate at the quick rate of day time.
only plants that are already under stress usually get sick from either decease or bugs, healthy plants can usually recover from this, the last thing you need to do is panic and this causes you take take instant action and then expect results the next day, you don't so, you rush out and try something else. As with all plants remedies take time to work, like us with medication, one spoonful don't cure the illness, it takes several days sometimes weeks, just be patient, if water is needed, it will take a whole week of watering daily to help, if it's disease it will take the same for a cure to work, don't pour stuff on the plants unless you know THIS IS the problem, it will end up killing your plants quicker.
Hope all this makes some sense and you get your lovely Lavender garden to bring you much joy.
Good luck. WeeNel.
