Nurse bees are suspected of transmitting the disease by carrying the virus from cell to cell. It is also believed that robber bees spread the disease by carrying

Relatively little is known about bee diseases caused by fungi. The two most significant fungus diseases are chalkbrood and stone-brood.

Figure 6. In sacbrood, the larva dies in an upright position. Left: The erect head of a diseased larva. Right: The head of a healthy larva.

Chalkbrood is caused by the contaminated honey from colony to colony.

Control

No chemotherapeutic agent is effective in preventing or controlling sacbrood. Requeening of colonies gives some degree of success. Generally, a colony will recover from sacbrood without a beekeeper's aid.

Fungus Ascosphaera apis. In 1968, the first case of chalkbrood was reported in the United States. Before then, it was solely a European problem. The disease has been reported in California, Minnesota, Montana, and North Dakota.

Young pupae or recently sealed larvae are the most susceptible to chalkbrood. Diseased bees are usually covered with filaments that have a fluffy, cottonlike appearance. If the fungus produces spores, the diseased brood will turn gray or, sometimes, black.

Chalkbood is seldom considered serious because it is not highly contagious. Colonies eliminate this disease without bee-keeper help.

Stonebrood is caused by the fungi belonging to the genus

Aspergillus, primarily A. flavus. These fungi can be found in soil, on accumulations of dead bees, and on honeycombs.

When larvae and pupae are infected by this disease they have a green, powdery substance on their bodies. Spores form earliest and most abundantly near the head end of the dead larvae.

After dead larvae and pupae become dry, they are known as mummies. The disease is called "stone brood" because of the hard texture of the dead brood.

Infection With Two Or More Diseases

When more than one disease affects a comb it is easy to overlook a mixed infection. For example, larvae infected by American foulbrood have been found in the same comb with larvae infected by European foulbrood or sacbrood.

No single larva, however, has been found to be infected with more than one disease. Even when another disease is known to be present, make a careful search for American foulbrood. This is important when disease samples are sent for diagnosis-be sure that the sample is representative of the diseased comb.

Noninfectious Diseases

Many beekeepers fail to recognize noninfectious diseases. Often, they assume that discolored larvae or pupae are the result of some infectious agent.

In the spring, dead adult bees and some larvae or young pupae are found at the entrance to the hives. This is not always indicative of a contagious disease; brood is often neglected because of a shortage of nurse bees and then dies from either chilling or starvation.

Typically, the center of the brood cells appear normal-only those larvae on the periphery of the combs appear abnormal. This colony condition could be caused by an infectious disease of adult bees or by toxic chemicals and is not necessarily a brood disease.

Poisonous plants can cause noninfectious diseases. Purple brood is a common disease of this type in the Southeastern United States. It is caused by either the pollen or nectar from the southern leatherwood, Cyrilla racemiflora L.