Dry rot describes a type of decay resulting in dry, shriveled, and mummified plant tissue. This condition is not simple desiccation, but the result of specific fungal or bacterial diseases that thrive in particular environmental conditions. The appearance of dryness is often a secondary symptom, occurring after the pathogen kills the plant cells and the tissue loses moisture. Understanding the causative agents, identification markers, and effective management strategies is necessary for controlling this widespread plant issue.
The Specific Causes of Dry Rot in Plants
Dry rot primarily affects storage organs and roots, caused by several species of fungi, most notably those in the genus Fusarium. These soil-borne pathogens require a wound to initiate infection in tubers, bulbs, or roots. Fusarium sambucinum frequently causes dry rot in stored potatoes, creating internal pockets of desiccated, dead tissue.
The fungus is a wound pathogen, unable to penetrate healthy, intact skin. Wounding occurs during harvest, handling, or through micro-injuries caused by soil pests like nematodes. Environmental stress, such as chronic underwatering, can also kill fine root hairs, providing F. solani dead tissue to colonize.
Dry decay can also be caused by Botrytis cinerea (gray mold), which affects stems, leaves, and fruits. This opportunistic fungus thrives under high humidity and poor airflow, leading to gray, fuzzy growth. Infected parts shrivel and dry out. Both pathogens survive in soil and plant debris, ready to infect a susceptible host.
Identifying the Distinct Symptoms of Dry Decay
The most telling sign of dry rot, especially in tubers and corms, is the formation of dark, sunken lesions on the exterior surface. As the underlying tissue dies and loses moisture, the skin above the lesion wrinkles in concentric rings, appearing shriveled. The lesion has a firm, dry texture, distinguishing it from the soft, mushy feel of bacterial soft rot.
Internal inspection reveals necrotic tissue ranging from light tan to dark chocolate brown or black. This dead tissue is characteristically dry, powdery, or corky, often forming hollow cavities inside the tuber. These internal spaces may be lined with the visible mycelium of the fungus, sometimes appearing as cottony tufts of white, yellow, or pink mold (spores).
Above-ground symptoms include wilting and premature leaf yellowing, indicating root or stem damage. Stems may develop dark cankers, and the affected foliage often becomes brittle and mummified instead of soft and slimy. This rapid desiccation of the affected plant part is the source of the “dry rot” moniker.
Immediate Management and Treatment Protocols
Managing an active infection requires the immediate removal and destruction of all visibly infected plant material to halt the spread of fungal spores. For storage crops, infected tubers or bulbs must be culled immediately to prevent contamination of the entire batch. Tools used for cutting diseased tissue must be sterilized after each use, typically with isopropyl alcohol or a weak bleach solution, to avoid transmitting the pathogen.
If roots are involved, carefully examine the root ball. All soft, dark, or mushy roots must be trimmed away with sterilized shears until only firm, healthy tissue remains. Repotting should use a fresh, well-draining potting medium, and the original container must be disinfected or replaced. After trimming, adjust the watering schedule, keeping the soil lightly moist rather than saturated to encourage new root growth.
Chemical treatments suppress fungal growth on seed pieces and stored crops or act as a preventative measure. Fungicide seed piece treatments, containing active ingredients like Mancozeb or Difenoconazole, are applied before planting to protect the cut surface from Fusarium infection. Post-harvest fungicide application may be used for stored produce, but growers must be aware of potential fungicide resistance in some Fusarium strains.
Long-Term Strategies for Prevention
Long-term prevention requires establishing an environment that discourages pathogen entry and proliferation. This starts with soil management, ensuring the use of porous, well-draining soil mixes amended with materials like perlite, pumice, or bark. Improving soil structure prevents waterlogging, which can suffocate roots and create anaerobic conditions favorable to secondary infections.
For garden beds, a three to four-year crop rotation schedule helps starve out soil-borne pathogens like Fusarium by avoiding repeated planting of host species. Selecting plant varieties with a higher degree of resistance to dry rot is a cultural control measure. Proper sanitation is essential, including cleaning planting areas and storage facilities before receiving new stock.
A particularly important practice for storage crops is post-harvest curing, which helps the plant heal minor wounds incurred during harvesting. For potatoes, this involves holding the tubers for one to two weeks in a warm environment (50°F to 55°F) with high humidity and good airflow. This controlled environment allows the tuber to form a protective layer of suberin over wounds, sealing off entry points required by dry rot fungi.